CLEAVAGE 



243 



the embryo and larva. After a variable but considerable 

 number of yolk pyramids are formed the planes of separation 

 gradually disappear except in the superficial protoplasmic 

 layer, which alone remains cellular, while the yolk again 

 becomes a solid mass. Such a process as this is taken to mean 

 that this type of cleavage may have been derived from the 

 total adequal type. 



FIG. 118. Cleavage in the beetle, Hydrophilus. From Korschelt and Heider, 

 after Heider. A, B. Intravitelline divisions of the nucleus. C. Beginning of 

 the formation of a superficial layer of cells. D. Later stage in formation of 

 "blastoderm," or superficial cell layer, b, blastoderm, or superficial layer of 

 cells; d, yolk; /, nuclei surrounded by protoplasm (protoplasmic islands) ; z, nuclei 

 remaining in yolk (merocytes). 



In other examples of nearly all groups of Arthropods the 

 yolk pyramids are incompletely developed or even entirely 

 absent and the cleavage is strictly superficial (Fig. 118). 

 Here, as in the preceding, blastomeres may be formed either 

 wholly or only partially around the ovum. The preliminary 

 divisions of the nucleus and the formation of protoplasmic 

 islands occur as described above. In many of the Insects, 

 whose cleavage is typically superficial, the substance of the 

 ovum ultimately becomes completely divided internally. 



The preceding classifications and descriptions of cleavage 

 have been almost wholly morphological in their basis. But 



