DEVELOPMENT OF ELASMOBRANCH FISHES. to 



almost the extreme posterior to the anterior end of the embryo. 

 It lies between the ventral wall of the spinal canal and the 

 dorsal wall of the intestine ; and round its posterior end 

 these two w^alls become continuous with each other {vide fig.). 

 Anteriorly the termination of the notochord cannot be seen, 

 it can only be traced into a mass of mesoblast at the base 

 of the brain, which there separates the epiblast from the hypo- 

 blast. The alimentary canal (cd.) is completely closed anteriorly 

 and posteriorly, though still wddely open to the yolk-sac in the 

 middle part of its course. In the region of the head it exhi- 

 bits on each side a slight bulging outwards, the rudiment of 

 the first visceral cleft. This is represented in the figure by two 

 lines (i V. c). The visceral clefts at this stage consist of 

 a pair of simple diverticula from the alimentary canal, and 

 there is no communication betw^een the throat and the ex- 

 terior. 



H. 



The present embryo is far larger than the last, but it has 

 not been possible to represent this increase in size in the 

 drawings. Accompanying this increase in size, the folding off 

 of the embryo from the yolk has considerably progressed, and 

 the stalk which unites the embryo with the yolk is propor- 

 tionately narrower and longer than before. 



The brain is now very distinctly divided into the three 

 lobes, whose rudiments appeared during the last stage. From 

 the foremost of these, the optic vesicles now present themselves 

 as well-marked lateral outgrowths, towards which there appears 

 a growing in, or involution, from the external skin (op.) to form 

 the lens. The opening of this involution is represented by the 

 dark spot in the centre. 



A fresh organ of sense, the auditory sac, now^ for the 

 first time becomes visible as a shallow pit in the external skin 

 on each side of the hind-brain (au. v.). The epiblast w^hich is 

 involuted to form this pit becomes much thickened, and 

 thereby the opacity, indicated in the figure, is produced. 



The muscle-plates have greatly increased in number by 

 the formation of fresh segments in the tail. Thirty-eight of 

 them were present in the embryo figured. The mesoblast at 

 the base of the brain has increased in quantity, and there is 



