203 



the periplieiy of the yolk mass now collects into a lenticu- 

 lar mass — the germinal disc. When the egg is floating 

 freely, the germinal disc lies downwards, the yolk being 

 uppermost. This lower part of the egg has been termed 

 the " animal pole," the upper part the " vegetative pole." 

 A few hours after fertilization meroblastic segmentation 

 begins, by the formation of a single vertical furrow which 

 divides the germinal disc into two blastomeres. This is 

 followed soon by a second furrow transverse to the first 

 and four equal blastomeres are formed. Vp to the 8-celled 

 stage at least there are no horizontal segmentation planes. 

 After this stage both vertical and horizontal division 

 j)lanes occur and the germinal disc segments with increas- 

 ing irregularity until it becomes a many-layered mass of 

 small cells. The blastoderm is thus formed. 



The rim of the blastoderm now begins to grow out- 

 wards and to envelop the yolk mass by a process of 

 epibolic gastrulation. Towards the end of the 2nd dayt 

 it has extended over the yolk so as to cover about 70° of 

 the latter when seen in optical section. The growing 

 margin of the blastoderm is slightly thickened. The 

 space enclosed within this blastodermic ring where the 

 yolk mass comes to the surface is the blastopore, and with 

 the growth of the blastoderm past the equator of the ovum 

 its area continiially diminishes. At first it is circular in 

 form and then becomes oval. After (i-T days from fer- 

 tilization it disappears entirely. 



The embrj^o begins to be raised off on the ord day 

 after fertilization. It lies in such a position that the 

 extremity which becomes the posterior one is situated 

 against the edge of the blastopore. On the 4th day it has 

 lengthened out considerably. The notochord and neural 



t Development is suiiposed to be effected at a temperature of about 



7°C. 



