242 GENERAL EMBRYOLOGY 



cleavage begins the nuclear structures are located centrally, 

 together with a small amount of protoplasm, and surrounding 

 this is a dense mass of yolk, interpenetrated by a very fine 

 network of protoplasm. At first nuclear division is not followed 

 by division of the inert remainder of the egg mass. But after 

 a varying number of nuclear divisions, the daughter nuclei 

 separate, each accompanied by a small mass of protoplasm, 

 the "protoplasmic island" and migrate to the surface of the 

 egg, continuing to multiply as they go (Figs. 117, 118). In 



D 



FIG. 117. Superficial cleavage in the Decapod, Dromia. (Sections.) After 

 Cano. A, B. Intravitelline divisions of the nucleus. C, D, E. Formation of 

 yolk-pyramids. F. Blastula; a superficial layer of cells enclosing a mass of 

 yolk, n, nuclei; p, yolk pyramids; y, yolk bodies. 



this way the nuclear and cytoplasmic portions form a kind of 

 superficial syncytium leaving the condensed and undivided 

 yolk centrally. 



Now either of two things may occur. In some forms (Deca- 

 pods, Copepods, Ostracods, Amphipods) cleavages appear 

 almost simultaneously, dividing the egg completely into a 

 number of cone-shaped cells with the apices directed centrally; 

 these cells are known as yolk pyramids (Fig. 117). In some 

 cases the formation of the yolk pyramids does not occur 

 simultaneously throughout the egg, but occurs earlier on that 

 side of the egg which corresponds with the ventral surface of 



