GERM LAYERS. 55 



merited cells, and beneath this a layer of less pigmented cells. Fig. 38 shows 

 the beginning of gastrulation, being a slightly earlier stage than the Triton 

 gastrula (Fig. 36). 



In the frog (also in the toad and salamander) a modification of the comple- 

 tion of gastrulation occurs, which, while apparently unimportant, is considered 

 by some investigators as having significance hi the interpretation of gastrulation 

 in higher forms, especially in Mammals. It is illustrated in Fig. 39. The 

 wedge-shaped mass of yolk cells is pushed in front of the invagination cleft and 

 carried around dorsally just beneath the ectoderm (Fig. 39, ft). This is met hi 

 the medial dorsal plane by yolk cells which have grown up from the floor of the 

 segmentation cavity on the opposite side (Fig. 39, c). What was the segmenta- 



Cells with 

 much pigment 



,^^^^^___ ^^^^^^^ Cells yrith 



Micromeres -M * ^B 5^ less pigment 



Macromeres 



Invagination (blastopore) 

 FIG. 38. From sagittal section of blastula of frog, showing beginning of gastrulation. Bonnet. 



tion cavity thus becomes divided into a cleft beneath the ectoderm and a cavity 

 surrounded by yolk cells. The cavity is designated by Bonnet the " Erganzungs- 

 hohle" or "completioti cavity" (Fig. 39, c, d t e). With continued enlargement 

 of the invagination cavity, the cleft-like remains of the segmentation cavity 

 beneath the ectoderm becomes obliterated and the "completion cavity" becomes 

 pressed ventrally. The wall between the latter and the invagination cavity 

 thins and finally ruptures so that the two cavities become one. 



It thus happens that at one stage there are three cavities (Fig. 39, d) (i) 

 the slit-like remains of the segmentation cavity, (2) the invagination cavity and 

 (3) the so-called "completion cavity." The remains of the segmentation 

 cavity is seen by reference to the figures to lie between the ectoderm externally 

 and the protentoderm and yolk entoderm internally. The invagination cavity 



