246 COMPARATIVE ZOOLOGY OF CHORDATES 



grows backwards and the embryo increases in length. In 

 Craniates, the quantity of yolk present prevents simple in- 

 vagination, and the rim of the blastopore arises as the result 

 of overgrowth (epiboly) accompanied by invagination or some 

 form of ingrowth. There is an increasing tendency for the 

 invagination to become reduced as the quantity of yolk in- 

 creases, and the yolk ceases to become encircled in the process 

 of closure of the blastopore. At the same time, the endoderm 

 appears early (one might say out of its turn), and the aperture 

 of the blastopore becomes virtual. 



Literature 



Assheton, R. Growth in Length. Cambridge University Press, 191 6. 

 Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon 

 Press, 1 91 3. 



