When it is borne in mind that the thymus arises just where epiblast and hypoblast unite together, it is, I think, conceivable that some few true epiblastic cells might get drawn into the proliferation of hypoblast. It is of such cells thus accidentally drawn in that I believe HASSALL'S corpuscles to be derivatives. They are structures which in any case appear comparatively late, and this circumstance, as well as their obvious epiblastic nature, lends support to the view of such a mode of origin. Morphologically they cannot possess any significance. The supposed Thymus of Petromyzon. In his paper DOHRN referred to a thymus in this form once only, and promises further information about it at a subsequent stage. Apparently this promise was forgotten or overlooked. The only posivite statement we possess emanates from STANNIUS who describes it as a paired organ behind the gills in the region of the heart. There can be no doubt to my mind that the structure here referred to is the degenerate pronephros, for it is exactly in this position that one finds this body in Myxine. Of course the supposition must be tested by observation. Above the gills of the adult lamprey, of the Ammocoete, and of very young larval forms no thymus is to be found. The search was a very disappointing one, for it was to be hoped that the condition of the organ in such lowly forms might throw more light upon its nature. At present I incline to the belief that the Marsipobranchii possess no well-defined thymus. Perhaps this stands in relation to the very different structure of their gills, which, moreover, here wide blood sinuses along their outer margins. The thymus of fishes is an organ derived from the hypblast of the gill-clefts. Its original elements become converted into leucocytes, which probably serve for the protection of the important organs of piscine life, the gills. With the disappearance of the gills above fishes and Perennibranchiate Amphibians it undergoes a restriction in the area of its formation and a transfer of its functions unto the service of other organs than those for whose behoof it was originally formed. Probably these functions remain the same as before, but they also experience a limitation in this respect that the organ becomes of service to the organism only during early life. The higher we ascend in the Vertebrate scale the more do its functions become restricted to the period of youth of the organism, the more are they usurped by other lymphforming structures in maturity. As the guardian of the respiratory organs the thymus is functionally relieved by the tonsils (palatine, pharyngeal, etc.) in the Sauropsida and Mammalia, the change in the organs of respiration. These views are in their very nature nothing more than probabilities, but as HATSCHEK sententiously remarks, "the results of Comparative Embryology have always only the value of probable conclusions, in exactly the same degree as those of Comparative Anatomy. The relative certainty depends in both cases only on the number of the premisses and on the exactness (Schärfe) of the conclusion). 1) B. HATSCHEK, Lehrbuch der Zoologie, 1888, p. 26. perlecting organ following that in the |