290 DEVELOPMENT OF ELASMOBRANCH FISHES. 



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 hypoblast. The alimentary 

 canal (#/.) is completely closed anteriorly and posteriorly, though 

 still widely open to the yolk-sac in the middle part of its course. 

 In the region of the head it exhibits on each side a slight bulging- 

 outwards, the rudiment of the first visceral cleft. This is repre- 

 sented 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 between the 

 throat and the exterior. 



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 proportionately 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 which 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 still a certain 

 mass of unsegmented mesoblast which forms the tail-swellings. 

 The first rudiment of the heart becomes visible during this 

 stage as a cavity between the mesoblast of the splanchnopleure 

 and the hypoblast (///.). 



