CLEAVAGE. 



45 



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

 crease 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 two very different types of cells each 

 confined to its own part of the segmenting cell mass. 



B 



Fig. 27. — Cleavage of the frog's egg. Morgan. 

 A, Eight-cell stage; B, beginning of sixteen-cell stage; C, thirty-two-cell stage; D, forty-eight-cell 

 stage (more regular than usual); E, F, G, later stages; H, I, formation of blastopore. 



Meroblastic Cleavage. 



(a) Superficial. — This form of cleavage is seen in the centrolecithal eggs 

 of Arthropods. These eggs (see p. 12) consist of a central mass of nutritive 

 yolk surrounded by a comparatively thin layer of protoplasm. The seg- 

 mentation 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 ap- 



