CYCLOSTOMATA. 



8 9 



ventral surface of the yolk (B). With the growth of the em- 

 bryo the yolk becomes entirely confined to the posterior part. 

 This 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 difference 

 between it and the embryos of other Fishes in the position of 

 the yolk is due in the main to the fact that the post-anal portion 

 of the tail is late in developing, and always small. As the 

 embryo grows longer it becomes spirally coiled within the egg- 

 shell. Before hatching the mesoblastic somites become distinctly 

 marked (C). 



The hatching takes place at between 13 21 days after 

 impregnation ; the period varying according to the temperature. 



During the above changes in the external form of the 



FlG. 41. FOUR STAGES IN THE DEVELOPMENT OF PETROMYZON. 



(After Owsjannikoff.) 



c. cephalic extremity ; bl. blastopore ; op. optic vesicle ; ait.v. auditory vesicle ; 

 br.c. branchial clefts. 



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 



