BLASTULA, GASTRULA, AND GERM LAYERS 347 



mass of slowly differentiating cells lying in the germ ring, 

 between the superficial ectoderm of the blastoderm and the 

 involuted shelf of endoderm (Fig. 158, F). The gastral meso- 

 derm is seen budding off laterally from the primitive streak 

 region, also between ectoderm and endoderm or even beyond the 

 region where the endoderm is found (Fig. 158, F). Posteriorly 



FIG. 160. Diagrams of the formation of the Teleost embryo by confluence of 

 the germ ring, and the growth of the germ ring around the yolk. From Kopsch. 

 A. In half-profile. B. In profile. 



of course the gastral mesoderm of each side becomes continuous 

 with the peristomial mesoderm, and as peripheral portions of the 

 germ ring, where the three layers are slowly differentiating, 

 are continually passing into the posterior end of the primitive 

 streak, it is clear that peristomial mesoderm is constantly 

 becoming gastral, merely through relative change of position, 

 not through any change in the mode of its formation or in its 

 relation to the other germ layers. 



The Elasmobranchs and Reptiles are again transitional in 

 that vestiges of enteroccelic grooves may be seen as shallow 

 and narrow longitudinal depressions, either side of the mid- 

 line, in the region of which the formation of mesoderm is most 

 rapid (Fig. 158, F). In the Teleost, as in the Bird, no traces of 

 enteroccels are to be seen. As usual the notochord forms from 

 the cell mass lying between the rudiments of the gastral meso- 

 derm, and may be said to have been derived either from the 

 gastral mesoderm, or from the endoderm in the same way that 



