242 COMPARATIVE ZOOLOGY OF CHORDATES 



and as it grows down to latitudes nearer the vegetative pole, 

 the diameter of the blastopore naturally decreases. Any given 

 point on the rim of the blastopore grows straight down along a 

 meridional line towards the vegetative pole ; but as the 

 diameter of the blastopore decreases, any two given points 

 on the rim at the start will find themselves closer together at 

 the finish of gastrulation. This process is called confluence. 

 In the case of the dogfish, the diameter of the blastopore (edge 

 of the blastoderm) has to increase considerably until it has 

 grown down and passed the equator of the yolk, whereupon 

 it decreases again. 



It is characteristic of these lower vertebrates (fish, frog, and 

 newts), that the rim of the blastopore arises along the margin 



yj> c 



A " 3> 



Fig. 114. — Views of the blastoderm of Hypogeophis, one of the Gymno- 

 phiona showing the origin and closure of the blastopore. (From 

 Jenkinson, after the brothers Sarasin.) 



The anterior edge of the blastoderm here does not become the ventral 

 lip of the blastopore, yp, yolk-cells seen through the blastopore. 



separating the protoplasmically-rich cells of the animal hemi- 

 sphere from the cells rich in yolk (or the undivided yolk) of 

 the vegetative hemisphere. Also, that in the closure of the 

 blastopore, the yolk should be enclosed by the growth of the 

 anterior part of this margin which becomes the ventral lip of 

 the blastopore. 



This is, however, not the case in the higher forms (reptiles, 

 birds, and mammals), in which there is a primitive streak. 

 In order to understand the evolution of the primitive streak 

 from the simple blastopore of the lower vertebrates, it is 

 necessary to consider the condition in the Gymnophiona, 

 which is more or less intermediate. The quantity of yolk in 

 the Gymnophionean egg brings about the formation of a 



