the migration of "Mesodermelementen" on which DOHRN laid such stress 1).

The difference which subsists between the thymus of the Mammal and that of Raja in the early development is simply this. In Mammals, according to KOELLIKER's observations and figures, the thymus cells become at first arranged in epithelial fashion, forming glandularlike tubes, where as in Raja and other fishes the similar cells, immediately on their birth from the parent epithelium take on the lymphoid characters which are only adopted at a later stage in Mammals.

MAURER 2) would appear to have also noted this, as I believe, fundamental fact, but he attached too little weight to it. It is possible to be too much influenced by one's knowledge of the normal cause of development in higher forms in studying lower ones.

MAURER states 3) "Die stark wuchernden Epithelzellen der ersten Anlagen nehmen lymphoides Aussehen an, gehen aber an der Grenze des Organs direkt in das Epithel der Kiemenhöhlenschleimhaut über etc. Von der Unterlage her wuchern Bindegewebszellen zunächst in geringer Anzahl ein, welche, allmählich das Organ durchsetzend, vorerst nur als Stützgewebe und Träger von Blutgefässen zu betrachten sind. Die Hauptmasse der Thymus wird dann immer noch von den lymphoid aussehenden Zellen der epithelialen Anlage gebildet. Erst nach mehreren Monaten fallen diese Zellen in ihren epithelialen Charakter zurück, indem ihre Proliferationsfähigkeit erschöpft ist (?). In gleicher Zeit brechen längs der Blutgefässe und Bindegewebszüge von umgebenden Bindegewebe lymphoide Zellen in grossen Massen in die Thymus ein und etabliren sich in einer intermediären Zone, wo sie Lymphfollikel bilden".

With the opening words of this citation agreement may be expressed, but at the same time protest must be entered against the use of the term "epithelial". If it only occurred in the first paragraph of the above its meaning might have reference only to the place of origin of the thymus cells, but when employed further on it be

1) In opposition to VON KOELLIKER's remarks on p. 880 of the "Entwickelungsgeschichte", DOHRN writes thus: "Diesen Aeußerungen gegenüber ist es vielleicht von Interesse, daß ich oben betonte, wie von Anfang an in die epithelialen Wucherungen eine fast gleich große Zahl von Mesodermelementen einwandern", p. 49. As already mentioned my observations do not support this.

2) F. MAURER, Schilddrüse und Thymus der Teleostier. Morphol. Jahrb., Bd. 11, 1885.

3) p. 170.

comes apparent that a different significance is attached to the term. The expression "lymphoid aussehende Zellen der epithelialen Anlage" would seem to indicate that MAURER does not regard these cells as true lymph-cells. They would have the appearance of lymph-cells, and possibly function as such for a time, only to be replaced, when their powers of proliferation were exhausted, by true lymphoid cells. According to him these latter burst into the thymus and set up lymphfollicles within it.

DOHRN describes a corresponding wandering in of lymph-cells at the very outset of the development 1).

I must express dissent from the statements of both observers. Indeed, all I have seen leads me to deny that any such migration takes place at any period. In the course of the work the thymus has been followed in its development from the very start to very advanced stages, in many of which it bears the closest resemblance in structure to a lymph gland.

Since in Raja batis one can follow the original epithelial cells from the moment they lose their epithelial characters on proliferation from the dorsal wall of the cleft, until and after the structure of the mature thymus is acquired 2), the conclusion to be drawn is naturally that the lymph elements of the thymus are the direct offspring of the epithelial cells of a gill-cleft. And this, as I understand him, is the view Professor VON KOELLIKER took 3).

In other words we have here the origin of a leucocyte-forming structure from the hypoblastic epithelium of gill-clefts.

1) GULLAND (Rep. Roy. College of Physicians. Edinb., Vol. III, 1891) also believes that the original epithelial foundation becomes substituted or replaced by adenoid tissues.

2) Except that no concentric corpuscles are yet present. The nature of these will be discussed subsequently.

3) Several weeks after the completion of my manuscript I found the recent instructive memoir of A. PRENANT in the current number of "La Cellule" (Contribution à l'étude du développement organique et histologique du thymus etc., T. 10, Fasc. 1, 1894). The paper treats of the development of the thymus of Mammals, i. e. the sheep, and the author, like myself, is led by his studies to confirm KOELLIKER'S statements as to the transformation of the original epithelial elements of the thymus into lymphoid cells (see p. 143). PRENANT's work affords a valuable and welcome complement to my own, for whereas he has concerned himself largely with a detailed account of the histological development, the morphological aspect of the thymus problem has been to myself of paramount interest. For further information reference must be made to the original.

A natural enquiry is then that as to what light this may throw on the functions of the thymus in fishes.

MAURER describes the organ as underjoing an involution and degeneration in adult life in fishes. This cannot be regarded as yet proved on as probable. He failed to find it in a few cases, but LEYDIG 1) speaks of the organ as though it were constantly present. In this connection I feel obliged to quote from a previous work of MAURER'S. After describing the position in which, according to LEYDIG and STANNIUS, the thymus of fishes was to be found, he says "Ein derartig gebautes und gelagertes Organ konnte ich bei den von mir untersuchten Fischen nicht nachweisen" 2).

Personally I have not as yet examined the organ in any Teleost, but among the great number of specimens of Raja which are annually dissected in the practical classes of this University, I have never found the slighest difficulty in recognising the presence of a thymus in any specimen.

The danger must be guarded against of expecting to meet with identical conditions in the history of the thymus of fishes and of Mammals. Differences in the initial stages of development have been referred to above.

If the organ be originally one in the service of the gills, one would certainly anticipate changes in those forms in which gills were no longer functional organs. In one very obvious respect this is certainly the case.

According to DE MEURON 3) the thymus of the lizard arises from three, that of the chick from one cleft. As great a reduction as in the latter case is encountered in Mammals. Here too the organ is of functional importance only during early life, and certainly for a much shorter period of life than in fishes, even if MAURER were admitted to be in the light as to its ultimate degeneration in the latter.

Probable Function of the Thymus.

The origin of leucocytes from the epithelium in the immediate neighbourhood of the gills can, I think, have only one meaning. They must be formed for the protection of the gills themselves. Possibly they serve for the devouring and removal of parts of the gills which

1) Lehrbuch der Histologie, 1857, p. 431.

2) F. MAURER, Morph. Jahrb., Bd. 9, p. 246.

3) DE MEURON, Recherches sur le développement du thymus et de la glande thyroïde. Receuil Zool. Suisse, 1886, T. 3.

have undergone necrosis, and of course their importance as guards against bacteria, spores of fungi etc. need not be dilated upon 1).

In this connection observations in other cases afford useful collateral evidence. Even as low down as the Coelenterata METSCHNIKOFF 2) has given reasons for a belief that special structures may exist for the removal of dead or diseased parts. On p. 5 of the separate copy we read "die sogenannten Nematocalyces (of Plumularia) würden somit als Organe aufgefaßt werden, denen vorzugsweise eine sozusagen prophylaktische Rolle zugeschrieben werden muß: sie fressen nekrotische Teile auf und belasten auch die benachbarten Organe, wahrscheinlich um die etwa vorhandenen schädlichen Stoffe in sich aufzunehmen und sie dadurch unwirksam zu machen."

It is out of question to recite here the general results of METSCHNIKOFF's unwearied investigations into the functions etc. of phagocytes and the rôle they play as guardians of the body. These results are the property of the whole scientific world. But it will be of interest and importance to quote one further passage from the memoir beforementioned. "Es scheint demnach, daß wohl im ganzen Tierreiche die wandernden Mesodermelemente ihre nahrungsaufnehmende und verdauende Thätigkeit zum Schutze des Organismus gegen Bakterien und solche Körper, welche einen günstigen Boden für deren Entwickelung bilden (nekrotische Teile), benützen. Bei Metazoen, welche noch kein entwickeltes Mesoderm haben, wird diese Rolle wahrscheinlich entweder durch Ektoderm (Plumularia) oder Entoderm ausgeführt" 3).

To my mind the thymus in its function bears some resemblance to the tonsils. Regarding these we possess SтÖHR's observations 4) on their nature and functions as guardians of the respiratory passages against the inroads of bacteria, fungus spores etc. According to him the tonsils are organs normally concerned in a "massenhafte Auswanderung lymphoider Zellen" into the oral cavity, the object of this being the removal of deleterious substances.

Lastly, there remains to notice an important piece of work by

1) The reduction of thymus-elements, even in fishes, has already been commented upon.

2) Untersuchungen über die intracelluläre Verdauung bei wirbellosen Tieren. In Arbeiten a. d. Zool. Inst. der Universität Wien, T. V, Hft. 2. The paging of the volume is not cited here.

3) p. 21.

4) PH. STÖHR, Zur Physiologie der Tonsillen, Biol. Centralblatt, 1882, Bd. 11, and other papers on the same subject. Of these a recent one (Anat. Anz., 1891, No. 19) deals with their development.

GUSTAV KILLIAN 1). This memoir closes with a long and interesting discussion which cannot be summarised without depriving it of much

of its cogency.

KILLIAN'S observations and reasoning and the latter seems to be convincing - show that in the "Gaumentonsillen" and "Rachentonsillen" we deal with structures concerned in the "Abwehr des Organismus gegen eindringende Schädlichkeiten, namentlich bakterieller Natur".

If the higher Vertebrates be provided with such organs in connection with their respiratory passages, would it appear strange if corresponding arrangements were met with in association with the gills of fishes ?

These are certainly very delicate organs, and by reason of their build structures liable to injury and desease of various sorts. No organs of fishes are more frequently the seat of parasites.

In default of a better explanation the thymus of fishes may therefore be regarded as a lymphoid organ specially differentiated from the gills themselves or from the gut-outgrowths which preceded the latter, for the protection of the gills in the ways above suggested.

HASSALL'S Corpuscles of the Thymus.

The consensus of opinion regarding these is that they are epithelial and epiblastic in nature. This may be at once conceded a superficial examination of sections of epitheliomata shows the close similarity between the two. DOHRN and MAURER both look upon these bodies as, in the words of MAURER, "von Epithelzellen gebildet, welche die Reste der epithelialen Anlage des Organes zum Teil darstellen" (p. 170). DOHRN (p. 51) goes much further than MAURER in this matter. As I unterstand him, these curious structures would represent the elements, or their descendents, which had been proliferated from the cleft. MAURER speaks more cautiously, and seems to incline to the opinion that they correspond to some only of such elements.

As yet I have met with no true and undoubted corpuscles in the thymus of the skate, but in sections from advanced embryos one sees here and there in the organ a cell (or a few) not of lymphoid nature but bearing a close resemblance to an ordinary epiblastic cell of the same animal.

1) GUSTAV KILLIAN, Ueber die Bursa und Tonsilla pharyngea. Morph. Jahrb., 1888, Bd. 14. A separate copy is quoted from.