EMBRYOLOGY OF TADPOLE AND CHICK 557 



inward (Fig. 323, C, en) and joins directly with the inner yolk cells to 

 form the entoderm. Due to their position, the inner yolk cells are also 

 entoderm, although they do not form by a true invagination. 



Viewing the entire egg externally during the process of gastrula- 

 tion, we may consider the germ-ring as something like a rubber band 

 placed about an ordinary ball in an equatorial plane. By sliding the 

 rubber band off the ball toward one side, we may understand how the 

 germ-ring brings its lateral region together in the mid-rim in the pos- 

 terior or caudal region. This coming together not only pushes the un- 

 derlying cells within, but causes the entoderm to extend further inward 

 and thus increases the cortical extent of the archenteron. 



We, therefore, have the first invagination of the pigmented cells 

 forming the dorsal lip of the blastopore; the invagination then extends 

 laterally in both directions to form the lateral lips of the blastopore; and 

 finally the process of invagination continues around to the side of the 

 gastrula, practically to a point almost opposite to where it began, and 

 where the ventral lip of the blastopore is formed. This completes the 

 entire blastopore. After the yolk plug has disappeared within the gas- 

 trula, the blastopore remains a narrow, elongated slit connected with 

 the archenteron. 



The method by which the blastopore grows by concrescence and 

 forms the primitive streak has already been described in the chick, a re- 

 reading of which should be done at this point. Comparisons should con- 

 stantly be made between the development of frog and chick embryos. 



It will be noted from what has been said that gastrulation is not so 

 much formed by invagination in the frog as it is by a delamination within 

 the gastrula itself. 



In fact, among the higher chordates, invagination is sometimes en- 

 tirely lacking, so that gastrulation may be accompanied by either involu- 

 tion, such as takes place in the chick, or by epiboly, which occurs to some 

 extent in the frog, and by delamination, a process just described. 



At this point Figure 325 must be studied to understand the varying 

 changes of positions of the blastopore brought about by the rotation of 

 the egg. It is also to be remembered that the blastopore marks the cau- 

 dal extremity of the embryo. 



The two-layered stage in the frog is of very short duration. 



In the inner region, where the germ-ring and the yolk-cells which 

 line the blastocoele are continuous, there are transitional cells which are 

 to become the mesoderm (Fig. 324, m). These cells are continuous with 

 ectoderm on one side and entoderm or yolk-cells on the other, and can 

 not be distinguished as definite mesodermal cells until the blastopore is 

 completely formed. 



In other words, the mesoderm first appears as a ring of cells just 

 within the margin of the blastopore. This mesodermal region is broad- 

 ened considerably in the dorsal region. 



