130 OUTLINES OF CHORDATE DEVELOPMENT 



certain other, commissures of the brain. The narrow depres- 

 sion between these two thickenings is the recessus opticus, i.e., 

 the passage to the cavities of the optic stalks. The posterior 

 side of the prosencephalon extends backward beneath the tip 

 of the notochord forming a well-marked outgrowth, the 

 irifnndibulnrn. 



FIG. 40. Median sagittal section through the brain of an embryo R. fusca, 

 of 2.3 mm. From Von Kupffer (Hertwig's Handbuch, etc.). cd, Notochord; 

 d, superficial layer of ectoderm ("deckschicht") ; en, endodermal lining of 

 pharynx; g, inner or nervous layer of ectoderm; hy, hypophysis; /, infundibulum; 

 kg, conical proliferation of ectoderm cells at the point of closure of the neural 

 folds. 



Somewhat later the entire dorsal wall of the prosencephalon 

 becomes thinner, and toward its posterior limit an evagination 

 appears which is the beginning of the epiphysis or pineal body 

 (Fig. 42). In front of this the roof ultimately becomes non- 

 nervous and forms a series of highly vascular folds projecting 

 down into the cavity of the brain; this is the choroid plexus of the 

 third ventricle. Later there develop, between this choroid 

 plexus and the epiphysis, the habenular ganglia and commissure, 

 and much later there develops, in front of this, a dorsal out- 



