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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. 



Fig. 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 Nemertine Linens, K. & EL, Fig. 

 105, and gastrulation in Apis, K. & 

 H., Fig. 495. 



III d . 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 

 former group of cases. 



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

 Fig. 22. A. Section of eye of chick embryo on third day. (See M., Fig. 413.) 



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

 lens layer; o. /. /., oviter lens layer ; r. c. retinal cup. 



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

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

 the latter. From H.-M., Fig. 69. 



Fig. 23. 



