GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 



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extent of the inequality varies in like manner. Taking the third 

 cleavage as a type, we may trace every transition from an equal divi- 

 sion (echinoderms, Polygordius\ through forms in which it is but 

 slightly marked (Amphioxus, frog), those in which it is conspicuous 

 (Nereis, Lymn&a, Polyclades, Petromyzon, etc.), to forms such as Clep- 

 sine, where the cells of the upper quartet are so minute as to appear 

 like mere buds from the four large lower cells (Fig. 123). At the 



Fig. 122. Cleavage of Nereis. An example of a spiral cleavage, unequal from the beginning 

 and of a marked mosaic-like character. 



A. Two-cell stage (the circles are oil-drops). B. Four-cell stage; the second cleavage-plane 

 passes through the future median plane. C. The same from the right side. D. Eight-cell stage. 

 E. Sixteen cells ; from the cells marked / arises the prototroch or larval ciliated belt, from X the 

 ventral nerve-cord and other structures, from D the mesoblast-bands, the germ-cells, and a part of 

 the alimentary canal. F. Twenty-nine-cell stage, from the right side; /. girdle of prototrochal 

 cells which give rise to the ciliated belt. 



extreme of the series we reach the partial or meroblastic cleavage, 

 such as occurs in the cephalopods, in many fishes, and in birds and 

 reptiles. Here the lower hemisphere of the egg does not divide at 

 all, or only at a late period, segmentation being confined to a disc- 

 like region or blastoderm at one pole of the egg (Fig. 124). 



Very interesting is the case of the tcloblasts or pole-cells character- 

 istic of the development of many annelids and mollusks and found in 

 some arthropods. These remarkable cells are large blastomeres, set 

 aside early in the development, which bud forth smaller cells in reg- 



