THE DEVELOPMENT OF AMPHIOXUS 39 



thickened by an accumulation of ganglion cells, and in its 

 posterior part two important structures appear. Just in front 

 of the opening of the cavity of the cord the ependymal cells 

 elongate forming a small median pad which is regarded as the 

 infundibulum (Fig. 13). Posterior to the cerebral vesicle 

 proper is an extensive dorsal diverticulum of the neurocoel : this 

 has been compared with the IV ventricle of Craniates. 



The simple character of the brain of Amphioxus is obviously 

 correlated with the general lack of special sense organs, par- 

 ticularly with the absence of the important optic and audi- 

 tory organs, and the feebly developed olfactory apparatus. 



2. The Gill Slits 



We may describe first the general morphological arrange- 

 ments of the developing gill slits: the development of the first 

 gill slit has already been mentioned. 



Although finally symmetrical and paired organs the gill 

 slits of the right and left sides develop independently, those of 

 the left side first. These appear successively in the mid- 

 ventral line, posterior to the first gill slit, and as they form 

 they shift upward, on the right side of the pharynx (Fig. 14). 

 These are called the primary gill slits, and when twelve to 

 fifteen, typically fourteen, have appeared, their formation 

 ceases. At this time the more anterior slits are large ovoid 

 openings lying well up on the right side; posteriorly they 

 diminish in size and lie toward the mid-line. In all these 

 early stages they are metameric and correspond with the 

 somites, a correspondence which is entirely' lost later on. 

 When the full number of fourteen primary gill slits has been 

 formed, the secondary gill slits, or those of the proper right 

 side, appear, also on the right side of the pharynx and dorsal 

 to the primary series. Their first indication is a longitudinal 

 ridge, in which appear six thickenings alternating with the 

 primary gill slits in the region between the third and ninth of 

 these (Fig. 14, B). 



The relations of these parts of the pharynx will become clear 



