A LI M EXT A R Y (J A XA L. 



627 



that the thyroid sometimes arises in the region of the first and sometimes 

 in that of the second cleft is probably to be explained by its rudimentary 

 character. 



The Thymus gland. The thymus gland may conveniently be dealt 

 with here, although its origin is nearly as obscure as ihs function. It has 

 usually been held to be connected with the lymphatic .system. Kolliker 

 was the first to shew that this view was probably erroneous, and he 

 attempted to prove that it was derived in the Rabbit from tlie walls of 

 one of the visceral clefts, mainly on the 

 ground of its presenting in the embryo an 

 epithelial character. 



Stieda (No. 569) has recently verified 

 KoUiker's statements. He finds that in the 

 I'ig and the Sheep the thymus arises as a 

 paired outgrowth from the epithelial rem- 

 nants of a pair of visceral clefts. Its two 

 lobes may at first be either hollow (Sheep) 

 or solid (Pig), but eventually become solid, 

 and unite in the median line. Stieda and 

 His hold that in the adult gland, the so- 

 called corpuscles of Ha^sall are the remnants 

 of the embryonic epithelial part of th(i gland, 

 and that the lymphatic part of it is of 

 mesoblastic origin ; but Kolliker believes 

 the lymphatic cells to be direct products 

 of the embryonic epithelial cells. 



The posterior visceral clefts in the course 

 of their atrophy give rise to various more 

 or less conspicuous bodies of a pseudo- 

 glandular nature, which have been chiefly 

 studied by Kemak'. 



Swimming bladder and lungs. A 



swimming bladder is present in all Ga- 

 noids and in the vast majority of Tele- 

 ostei. Its development however is only 

 imperfectly known. 



In the Salmon and Carp it arises, as was first shewn by Von Baer, 

 as an outgrowth of the alimentary tract, shortly in front of the liver. 

 In these forms it is at first placed on the dorsal side and slightly to 

 the right, and grows backwards on the dorsal side of the gut, between 

 the two folds of the mesentery. 



The absence of a pneumatic duct in the Physoclisti would appear 

 to be due to a post-larval atrophy. 



yyi. 



Fig. 417. Section through 

 the head of an elasmobranch 

 embryo, at the level of the 

 auditory involution. 



Th. rudiment of thyroid body ; 

 aup. auditory pit; «?<». ganglion 

 of auditory nerve; iv.v. roof of 

 fourth ventricle; a.c.v. anterior 

 cardinal vein ; aa. aorta ; I.ai, 

 aortic trunk of mandibular arch; 

 pp. head cavity of mandibular 

 arch; Ivi'. alimentary pouch which 

 will form the first visceral cleft. 



His and Kolliker. In view of the comparative development of this organ it is difficult 

 to accept either Wolfler's or Stieda's account. Wolfler's attempt to explain the sup- 

 posed errors of his i^redecessors is certainly not capable of being applied in the case of 

 Elasmobranch Fishes, or of Petromyzon ; and I am inclined to think that the method 

 of investigation by transverse sections, which has been usually employed, is less liable 

 to error than that by longitudinal sections which he has adopted. 



^ For details on these organs vide Kolliker, Entwicklun(js[ieschir]ite, p. 881. 



40—2 



