190 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
6. The former process occurs typically in the ontogeny of reptiles, and 
perhaps of mammals, where the middle of the layer of “primary ento- 
derm " roofing the yolk cavity atrophies, making a great hole in the layer. 
Fra. 22. 
7. The latter process is represented in Hatschel's familiar figure of 
the formation of the neural tube of Amphioxus (Figure 23), where the 
medullary plate is shown as falling 
og below the general level without the 
intervention of a fold. Compare the 
formation of the “body plate” of 
the Nemertine Lineus, K. & H., Fig. 
105, and gastrulation in Apis, K. & 
H., Fig. 495. 
III*. Finally, we must consider the 
important processes involving two or 
more protoplasmie layers. Such layers 
may move either towards or from each 
— 
Fic. 93. other. We will here consider only the 
the latter. From H.-M., Fig. 69, 
