1 66 



DEVELOPMENT OF AMPHIOXUS. 



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

 stead they begin to undergo a retrogressive development 

 and eventually flatten 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 Ammoccetes, 

 to show the relation of the first pair of gill-pouches to the peripharyngeal grooves. 

 (After DOHRN.) 



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



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



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

 I-VIIl. Gill-pouches, b.w. Body-wall, oes. Oesophagus, ph. Pharynx. 



ph.g. Peripharyngeal groove, st. Stomodoeum. vel. Velum. 



As to their position, they occupy the extreme anterior 

 angles of the pharynx formed by its lateral walls with the 

 anterior transverse wall against which the Stomodoeum is 

 applied. Whatever may be the reason for it, the atrophy 

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

 cisely the same nature as the atrophy of the first gill-slit 

 in Amphioxus, with the distinction that the latter actually 

 opens to the exterior for a time. 



