80 GENERAL SKETCH OF 



yolk-sac. It is below the second layer that the pigment- 

 corpuscles occur. 



The enlarged anterior part of the neurochord forms the 

 brain. It is rounded above, deeply carinate below, arched over 

 by the epiblastic integument and limited ventrally by the 

 hypoblast. From the anterior region of the brain, or fore-brain, 

 grow out the paired optic vesicles, and the mid-brain is easily 

 distinguished by its greater breadth and volume. Immediately 

 posterior to it is the hind-brain which passes into the neuro- 

 chord. It is remarkable that the brain forms nearly ^ of 

 the total length of the embryo in its early condition. 



When a quarter of the yolk is enveloped, a cleft appears on 

 each side of the post-optic region-. 



One anterior portion, the united mid- and fore-brain, can 

 now be distinguished from the hind-brain, which is soon 

 separated by a similar slight fold from the spinal cord behind. 

 The mid-brain, lastly, is constricted off by an interorbital fold 

 and the three regions become clearly defined. 



Instead of the medullary folds in the other vertebrates, 

 which close together and form the spinal cord with its median 

 cleft (neural canal), we have in these bony fishes a solid mass or 

 rod of cells forming the neurochord, which later acquires a 

 cavity by a separation of the median cells. In some Teleo- 

 steans, for example the gurnard, a remnant of the medullary 

 groove is seen in the deep temporary groove in the dorsal 

 surface of the neurochord about the fourth day whilst still in 

 the egg. 



In the cod, soon after the closure of the blastopore, a 

 fine cleft appears by separation of the median cells in the brain 

 along a vertical longitudinal plane. It commences in the 

 mid- brain and thence extends into the fore-brain almost to the 

 anterior limit of the latter. This is the first indication of 

 the true neural canal. Then two lateral continuations form a 

 cruciform fissure, marking off the fore-brain ; whilst a T-shaped 

 lumen is found in the mid-brain, the roof being thinner than 

 the walls or floor. The canal rapidly extends into the neural 

 cord in the trunk, giving off a pair of (vertical) lateral 

 cavities, forming the optic ventricles and iter a tertio ad quartiim 



