CYCLOSTOMATA. 



73 



surface or of the yolk (B). With the growth of the embryo 

 the yolk becomes entirely confined to the posterior part. This 



Fig. 41. Four stages in the development of Petromyzon. 

 (After Owsjanuikoff.) 

 c. cephalic extremity; hi. blastopore; op. optic vesicle; au.v. auditory vesicle; 

 br.c. branchial clefts. 



part is accordingly greatly dilated, and might easily be mistaken 

 for the head. The position of the yolk gives to the embryo a very 

 peculiar appearance. The apparent diti'erence between it and the 

 embryos of other Fishes in the position of the yolk is due in the 

 main to the fact that the postanal portion of the tail is late in 

 developing, and always small. As the embryo grows longer it 

 becomes spirally coiled within the eggshell. Before hatching the 

 mesoblastic somites become distinctly marked (C). 



The hatching takes place at between 13 — 21 days after im- 

 pregnation ; the period varying according to the temperature. 



During the above changes in the external form of the embryo, the 

 development of the various organs makes great progress. This is 

 especially the case in the head. The brain becomes distinct from the 

 spinal cord, and the auditory sacks and the optic vesicles of the eye 

 become formed. The branchial region of the mesenteron becomes 

 established, and causes a dilatation of the anterior part of the body, 

 and the branchial pouches grow out from the throat. The anus 

 becomes formed, and a neurenteric canal is also established (k^cott). 

 The nature of these and^other changes will best be understood by a 

 description of the structure of the just-hatched larva. The general 

 appearance of the larva immediately after hatching is shewn in fig. 

 41, D. The body is somewhat curved; the posterior extremity being 



