144 GENERAL EMBRYOLOGY 



selves, as in some Elasmobranchs, where the chromosomes at this 

 time shrink to about one-fortieth of their previous length and 

 one-tenth their previous diameter (Riickert, Fig. 34). Or the 

 "nucleolus" of the oocyte may be a karyosome or chromatin 

 nucleolus, and in such cases (Echinoderms, for example) during 

 synizesis the chromatin may be nearly all contained within this 

 body. Then the chromosomes are formed singly or in groups 

 out of this " chromatin reservoir." After they have all been 

 given off, much the greater part of the chromatin still remains 

 in the karyosome, which then may fragment before dissolu- 

 tion, or it may be dissolved directly (Fig. 35). The subsequent 

 behavior of the chromosomes is closely similar to that of the 

 spermatocyte chromosomes; tetrads may or may not be 

 formed, according to the species, as the chromosomes pass into 

 the division figure (Figs. 77, 78). The centrosome divides and 

 the spindle and asters form typically in most respects save in 

 size and position. The spindle is very small and in most eggs 

 is close to the surface of the cell at its animal pole (Fig. 76). In 

 alecithal and isolecithal eggs the nucleus and spindle are at 

 first located centrally and then later move to the periphery. 

 At first the spindle lies parallel to a tangential plane, but during 

 the mesophase it rotates through ninety degrees, putting its 

 axis in a radial direction (Figs. 76, 77). In many cases the 

 division of the oocyte is inhibited at this stage, until after the 

 entrance of the spermatozoon, when it proceeds to completion; 

 or this heterotype division may proceed without any inter- 

 ruption and the primary oocyte cut at once into two cells. The 

 extremely eccentric position of the nucleus in this stage leads to 

 one of the most characteristic features of oogenesis, namely, the 

 very unequal division of the cell body. One of the products of 

 division, the secondary oocyte, is of practically the same size as 

 the primary oocyte; the other cell the first polar body is 

 very much smaller, indeed usually minute (Figs. 76, 77, 78). 

 In essentials these two daughters of the primary oocyte are 

 equivalent; their nuclei are alike in size and composition, each 

 contains a daughter centrosome, but with the polar body there 

 is only the smallest amount of cytoplasm and practically none 



