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the periphery 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 dise into two blastomeres. This is 
followed soon by a second furrow transverse to the first 
and four equal blastomeres are formed. Up to the 8-celled 
stage at least there are no horizontal segmentation planes. 
After this stage both vertical and horizontal division 
planes 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 continually diminishes. At first it is circular in 
form and then becomes oval. After 6-7 days from fer- 
tilization it disappears entirely. 
The embryo begins to be raised off on the 5rd 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 
+ Development is supposed to be effected at a temperature of about 
ie 
