438 JOHN BEARD, 



at the ventral side of the spiracular pouch, but, on the contrary, at 

 its dorsal anterior portion. The reason it was termed ventral by Van 

 Bemmelen was to distinguish it from a more dorsally-lying body in 

 certain Elasmobranchs. On p. 174 he gives an account of its structure 

 and position. It has, according to him, the form of an oval vesicle, 

 made up of high columnar cells, and which is connected with the wall 

 of the spiracle by a narrow stalk, either solid or with a narrow lumen. 

 Its position in several Elasmobranchs is recorded. The period of its 

 appearance is stated to be that, at which cartilage first begins to be 

 formed in the embryo. Its development as an outgrowth of the dorsal 

 wall of the spiracular cleft is described. As will be seen, I have 

 nothing to add to this portion of Van Bemmelen's account, which so 

 far as it goes is quite correct. He noted, that it appeared to undergo 

 little change at later periods, and that in some forms it probably 

 disappeared, certainly this was so in Acanthias. 



As to its nature, on p. 177 he writes: "Dass das Bläschen ein 

 rudimentäres Organ ist, scheint mir unzweifelhaft Seine Ent- 

 stehungsweise hat erstens gewiss einige Aehulichkeit mit den ersten 

 Entwicklungsstadien der Thymuswucherungen an den übrigen Kiemen- 

 spalten." This comparison he then proceeds to justify by pointing 

 out resemblances in mode of development between the structures in 

 question. But, finding important difierences, he finally decides, that 

 the thymus-nature of the spiracular follicle is improbable, and further 

 on he inclines to regard it as a rudimentary gill-cleft. 



Although in my first paper upon the thymus the actual existence 

 of a spiracular thymus could not be established, it was recognised, 

 that probably Van Bemmelen's follicle represented such a body. It 

 was written there: "I incHne, certainly, to the view, which would 

 consider both the vesicular follicle of the spiracle and that of the 

 angle of the mouth as possibly the rudimentary equivalents of thymus- 

 elements of these parts. At any rate, the whole history of these 

 structures harmonises with this supposition" (1894, p. 478). 



With the writer the quest after the spiracular thymus in the 

 course of the present research lasted long before resulting in any- 

 thing very definite. While investigating the early development of the 

 gill-clefts and associated structures, the spiracle itself often came under 

 review. And it appeared remarkable, that, while its origin and much 

 of its later history were those of a typical gill-cleft and arch, while 

 what appeared to be the equivalent of a thymus-placode was easily 

 defined in embryos of 6 mm and upwards, in later periods no evidences 



