212 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



the latter grow down over the others at that side of the egg 

 which is later to form the hinder end of the embryo. This 

 downward growth results in the infolding of a portion of the 

 cells as a sort of pocket, the cells on the upper surface of the 

 pocket being small, those on the lower side the larger yolk-laden 

 cells. This pocket is the archenteron ; its walls are entoderm, 

 and its opening is the blastopore. As the process continues, the 

 blastoporal lips of either side come to meet in the median line, 

 producing the primitive streak and groove already described. 



In the elasmobranchs the process of gastrulation is still far- 

 ther modified by the large amount of unsegmented yolk. Here 



A B 



FIG. 218. Diagram of two stages in the formation of the embryo in an elasmo- 

 branch egg ; the inflected rim of the blastoderm divided up into segments so as to 

 illustrate the formation of the embryo by concrescence. 



the blastoderm forms a small circular patch of cells resting upon 

 the yolk. At the morphologically hinder portion of this blasto- 

 derm the cells begin to turn in between the rest and the yolk, 

 thus differentiating ectoderm and entoderm. As the blastoderm 

 increases in size by continual cell division plus additions from 

 the yolk, this infolded rim grows together, its halves swinging 

 in toward the middle line so that the grooves of either side unite 

 to form a tube, the archenteron, the floor of which is formed 

 by the yolk. The rim of the inflected tissue must be regarded 

 as the lips of the blastopore, and as these lips unite in the median 

 line they give rise to the primitive streak. 



