1 66 



DEVELOPMENT OF AMFHIOXCS. 



it does not come to a fusion with the ectoderm ; but in- 

 stead they begin to undergo a retrogressive development 

 and eventually tlatten completely out (Fig. 92 B). They 

 are thus shown to be rudimentary structures, morphologi- 

 cally representing the first pair of gill-clefts, but never 

 achieving their full development. 



Fig. 92. — Horizontal sections through the pharyngeal region of Ammocoetes. 

 to show the relation of the first pair of gill-pouches to the peripharyngeal groo%-es. 

 (After DOHRN.) 



A. Two days after hatching ; first pair of gill-pouches well developed. 



B. Si.x days after hatching; first pair of gill-pouches flattened out. 



C. Nine days after hatching; appearance of peripharyngeal grooves. 

 1~\'III. Gill-pouches, b.iv. Body -wall. I'c-j. CEsophagus. /-:. Pharyn.v. 



ph.^. Peripharyngeal groove, st. Stonrodceum. : el. \'elum. 



As to their position, thev occup^• the extreme anterior 

 angles of the pharvnx formed b}' its lateral walls with the 

 anterior transverse wall against which the stomodoeum is 

 applied. \\'hatever may be the reason for it, the atrophy 

 of the first pair of gill-pouches in Ammocivtes is of pre- 

 cisely the same nature as the atrophy of the first o-ilPslit 

 in Amphioxus, with the distinction that the latter actualh- 

 opens to the exterior for a time. 



