44 



TEXT-BOOK OF EMBRYOLOGY. 



only up to this point, while beyond this point the protoplasmic cells increase in 

 number much more rapidly than do the yolk cells, so that when the protoplasmic 

 cells number 128, there are still but comparatively few yolk cells. There thus 

 result in total unequal cleavage, cells of two very different sizes each confined 

 to its own part of the segmenting cell mass. 



Meroblastic Cleavage. 



(A) SUPERFICIAL. This form of cleavage is seen in the centrolecithal eggs 

 of Arthropods. These eggs consist of a central mass of nutritive yolk sur- 



c d 



FIG. 28. Cleavage in hen's egg. Coste. Germinal disk and part of yolk, seen from above. 



rounded by a comparatively thin layer of protoplasm. The segmentation 

 nucleus lies in the middle of the nutritive yolk where it undergoes the usual 

 mitotic divisions. The resulting daughter nuclei leave the central yolk mass 

 and pass out into the peripheral layer of protoplasm where they apparently 



