CH. VI] FORMATIOX OF THE GERM-LAYERS 65 



with the inner region of large yolk-cells. This ring of cells, 

 as subsequent development shows, is the beginning of the embryo, 

 and the ring itself is composed of the material which subsequently 

 forms the central nervous system, the mesoderm, the notochord, and 

 a part of the endoderm. An understanding of the subsequent 

 development depends on a knowledge of the changes that take 

 place in this ring. 



The material of the ring is intimately involved in the move- 

 ments that take place during the overgrowth of the lower 

 hemisphere by the lips of the blastopore. During this period, 

 we must picture to ourselves the ring as rising up and drawing 

 together over the lower white hemisphere, so that ultimately 

 it leaves its equatorial position and its halves come together to 

 form the embryo. (Fig. 24, A, B, C.) 



ABC 



FIG. 24. Diagrams illustrating germ-ring and concrescence of lips of blastopore. 



As the dorsal lip of the blastopore progresses over the white 

 hemisphere, its progress is due to the movement and fusion 

 along a meridian of the material of the equatorial ring. We 

 are to think of the material of the ring as moving toward the 

 middle line from the right and left sides (for with the estab- 

 lishment of the dorsal lip the ring becomes bilateral) and 

 fusing continuously in the dorsal lip (Fig. 24). The advance 

 of the blastopore is merely the expression of the absorption 

 into its dorsal lip of the material of the two sides of the ring. 

 As soon as the material from the sides reaches the median line 

 in the dorsal lip of the blastopore, it remains stationary and 

 new material is added behind that just laid down. The mate- 

 rial of the equatorial ring is thus carried into a meridian of 

 the egg. With the disappearance of the yolk-plug below the 



