DEVELOPMENT OF ELASMOBEAXCH FISHES. 225 



Dr Miiller^ in his exhaustive memoir on the thyroid body, 

 gives an account of its condition in two Acanthias embryos. In 

 his earliest embryo (which, judging from the size, is perhaps 

 about the same age as my latest) the thyroid body is discon- 

 nected from the throat, yet contains a lumen, and is not divided 

 up into lobules. It is clear from this account, that there must 

 be considerable differences of detail in the development of the 

 thyroid body in Acanthias and Scyllium. 



In the Bird Dr Muller's figures shew that the thyroid body 

 developes in the region of the hyoid arch, whereas, in Elasmo- 

 branchs, it developes in the region of the mandibular arch. 

 Dr Gotte's^ account of this body in Bombinator accords very 

 completely with my own, both with reference to the region in 

 which it developes, and its mode of development. 



TJie pancreas. 



The pancreas arises towards the close of stage K as a some- 

 what rounded hollow outgrowth from the dorsal side of that part 

 of the gut which from its homologies may be called the duodenum. 

 In the region where the pancreas is being formed the appearances 

 presented in a series of transverse sections are somewhat com- 

 plicated (PL XVII. fig. 1), owing to the several parts of the gut 

 and its appendages which may appear in a single section, but 

 I have detected no trace of other than a single outgrowth to 

 form the pancreas. 



By stage L the original outgrowth from the gut has become 

 elongated longitudinally, but transversely compressed: at the 

 same time its opening into the duodenum has become somewhat 

 narrowed. 



Owing to these changes the pancreas presents in longitudinal 

 and vertical section a funnel-shaped appearance (PL xviii. fio-. 4). 

 From the expanded dorsal part of the funnel, especially from 

 its anterior end, numerous small tubular diverticula grow out 

 into the mesoblast. The apex of the funnel leads into the duo- 

 denum. From this arrangement it results that at this period 

 the original outgrowth from the duodenum serves as a recep- 

 tacle into which each ductule of the embryonic gland opens 



1 Jenaische Zeitschri/t, Vol. vi. 



2 EntwicTihuKjsgeschichte d. Unke. 



