CELL-ORGANS OF SEA-URCHIN EGG 579 



somes. It appears homogeneous and achromatic and if it still, 

 contains the substance of chromosomes, this substance is chem- 

 ically changed and is no more chromatin, which once formed the 

 chromosomes. Moreover, if the number of chromosomes be 

 doubled at every mitosis without loss of size the amount of the 

 chromatin must be doubled. In any event we have to conclude 

 that the synthesis of chromatin, even if it does take place within 

 the nucleus, must be effected at the expense of a material derived 

 from the cytoplasm. And the facts of direct observation speak 

 strongly for resorption of definite basophilic chromatic substance 

 by the nucleus from the cytoplasm. 



This absorption can lead at different stages to various mor- 

 phological structures. Groups of small basophilic granules may 

 accumulate around the nucleus, and become connected with the 

 granules within the nucleus by threads; they seem to discharge 

 their substance into the nucleus; again cytoplasmic condensa- 

 tions appear around the nucleus and chromatin particles arise 

 in the nucleus in the form of innumerable small granules, in the 

 immediate vicinity of the cytoplasm ; or finally the resorption by 

 the nucleus does not proceed over the whole periphery of the 

 nucleus, but is chiefly localized at one or two points of it and a 

 considerable body of chromatin is formed, (see figures 8, "9, 10, 

 11, 12 and 13). The nucleus often remains spherical, the radia- 

 tion around the nucleus is single and no centrosome is seen in 

 eggs while the resorption is proceeding over the whole periphery 

 of the nucleus. 



More complicated is the differentiation of the chromatin in 

 the nucleus during normal fertilization and more difficult is its 

 study. The process of chromatin development in a normal egg 

 is more complex because of the copulation of the pronuclei, at 

 which time the segmentation nucleus usually receives a consider- 

 able amount of differentiated chromatin from the spermatozoon. 

 The copulation of the pronuclei can procceed in different ways. 

 Sometimes the pronuclei join together very quickly and the 

 chromatin of the spermatozoon is added to the female pronucleus 

 entirely and unchanged (fig. 14) ; again the spermatozoon, before 

 the union with the female pronucleus, may change into an achro- 



