190 



BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



,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. 



P^iG. -22. 



7. The latter process is represented in Hatschek's familiar figure of 

 the formation of the neural tube of Amphioxus (Figure 23), where the 



medullary plate is shown as falling 



below the general level without the 



intervention of a fold. Compare the 



formation of the " body plate " of 



the i^emertine Linens, K. & H., Fig. 



105, and gastrulation in Apis, K. & 



H., Fig. 495. 



IIP. Finally, we must consider the 



important processes involving two or 



more protoplasmic layers. Such layers 



may move either towards or from each 



„ other. We will here consider only the 



Fig. 23. ^ "^ 



former group of cases. 



8. The mutual approach and fusion of layers may be called con- 

 Fig. 22. A. Section of eye of chick embr^-o on third day. (See M., Fig. 413.) 



B. Section of eye of rabbit embryo, 13 days old. (See M., Fig. 409.) i. I. A, inner 

 lens layer; o. I. /., outer lens la^^er ; r. c. retinal cup. 



Fig. 23. Section across an Amphioxus embryo, showing tlie medullary plate, 

 m. p., whicli has sunk below the level of the ectoderm, producing a discontinuity in 

 the latter. From U.-M., Fig. 60. 



