MICRODISSECTION STUDIES. 



345 



nation they were pinched off from the cortical portion of the eggs. 

 None developed of those which were separated in such a way that 

 there was no question as to their lacking any of the original cortex 

 of the egg. 



An endoplasmic sphere, in order to develop at all, apparently 

 must incorporate in its substance at least a part of the original 

 cortex of the egg. This is shown in Fig. 27. An exovate was 



3. IS 



11.15 



FIG. 27. a, an exovate is produced l}y squashing and most of the ecto- 

 plasmic part is cut away along line of arrow, b, the endoplasmic sphere formed 

 itself incorporating a small part of the cortex. Upon fertilization the small 

 cortical region formed a partial fertilization membrane, c, many furrows 

 form simultaneously over the surface of the egg showing that it has been 

 fertilized. (Note that the small cortical piece to one side of the egg has 

 segmented in two.) d, the egg has reverted into a multinucleated nonseg- 

 mented mass except for three blastomere-like bodies which were pinched off. 

 e, the fragment is again attempting to segment. 



produced by crushing an egg (Fig. 27-0). However, before the 

 exovate was set free most of the cortical remnant was cut away, 

 leaving a very small piece which was drawn into the circumference 

 of the endoplasmic sphere. On being inseminated a small shred 

 of the egg membrane lifted off from this remnant, and this was 

 all that constituted the fertilization membrane (Fig. 2/-&). A 

 sperm on entering this sphere underwent nuclear division several 

 times. This was followed by cleavage furrows which formed on 

 the surface of the egg between the peripheral nuclei and gave to 

 the egg the appearance of a mulberry (Fig. 2/-c). Some of the 

 furrows deepened sufficiently to pinch off nucleated bodies. A 

 few minutes later the furrows became obliterated and the main 

 body of the egg appeared again as a non-segmented but multi- 

 nucleated mass (Fig. 2"/-d). This process may occur several 

 times (Fig. 27-^). The ability of an exovate to approximate 

 normal segmentation is a function of the amount of the original 

 egg cortex which it incorporates. 



