A LI ME NT A R Y CA XA L. 



>_ < 



iv.r 



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 glaud may conveniently be dealt 

 with here, although its origin is nearly as obscure as its function. Tt 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 the walls of 

 one of the visceral clefts, mainly on the 

 ground of its presenting in the embryo an 

 epithelial character. 



Stiecla (No. 569) has recently verified 

 Kolliker's statements. He finds that in the 

 Tig 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 Hassall are the remnants 

 of the embryonic epithelial part of the 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- 



T/i 



FIG. 417. SECTION THROUGH 



THE HEAD OF AN ELASMOBBANCH 

 EMBRYO, AT THK LEVEL OF THE 

 AUDITORY INVOLUTION. 



i-i " i / Tli. rudiment of thyroid body; 



glandular nature which have been chiefly mip , auditory pit; ,/. gallglim ; 



studied by Kemak . of auditory nerve; iv.r. roof of 



~ . . . , . . . fourth ventricle; a.c.v. anterior 



Swimming bladder and mngS. A cardinal vein; . aorta; I.a.r. 



swimming bladder is present in all Ga- aortic trunk of mandibular arch; 



noids and in the vast majority of Tele- pp \ bt? 7 ad c vit y of raan 1 dlb la 1 r 



J J arch; l-vc. alimentary pouch which 



ostei. Its development however is only w m form the first visceral cleft, 

 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 mesenterv. 



> 



The absence of a pneumatic duct in the Physoclisti would appear 

 to be due to a post-larval atrophy. 



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 predecessors is certainly not capable of being applied in the case of 

 Elasmohranch 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. 



1 For details on these organs vid>- Kolliker, Entwicklungsffeachichte, p. 881. 



402 



