33 



The neural canal now differentiates into a tubular dorsal 

 nervous system. The anterior end dilates to form the 

 thin-walled cerebral vesicle (fi<::js. 8 and 9, n.v.), containing 

 later the intra-cerebral, dorsal, pigmented eye (oc.) and the 

 ventral otolith {an.) of the larva. The next part of the canal 

 thickens to form the trunk ganglion, and behind that is 

 the more slender spinal cord {n.c), which runs to the extre- 

 mity of the tail. A ciliated diverticulum of the anterior 

 end of the enteric cavity (future pharynx), which enters 

 into close relations with the front of the cerebral vesicle, 

 and later opens into the ectodermic invagination which 

 forms the mouth at that spot, is evidently the rudiment 

 of the neural or hypophysial gland and canal. 



The future branchial sac (fig. 9, mes.), with a ventral 

 median thickening which will be the endostyle, is by this 

 time clearly distinguishable, by its large size, from the 

 much narrower posterior part of the enteron which grows 

 out to become the oesophagus, stomach, and intestine. 



The notochord does not extend forward into the 

 branchial region, but is confined to the posterior or 

 caudal part of the embryo. It now shows lenticular 

 pieces of a gelatinous intercellular substance secreted by 

 the cells and lying between them (fig. 8). The mouth 

 forms as a stomodseum or ectodermal invagination antero- 

 dorsally, in the region where the neuropore had closed 

 up ; and, about the same time, two lateral ectodermal 

 involutions appear (fig. 9, at.) which become the atrial or 

 peribranchial pouches, at first distinct, afterwards united 

 in the mid-dorsal line to form the adult cloaca and atrial 

 aperture. 



In-growths from the atrial pouches and out-growths from 

 the wall of the pharynx coalesce to form the proto- 

 stigmata (primary gill slits) by which the cavity of the 

 branchial sac is first placed in communication with the 



