142 GENERAL EMBRYOLOGY 



The divisions of the oogonia are normal ; the diploid number 

 of chromosomes appear, and the details of spindle, aster, and 

 centrosome, call for no special mention. Aside from the 

 chromosomal behavior, the divergences of the later maturation 

 divisions from the normal are partly the result of the enormous 

 growth of the egg cell, and partly in the nature of adaptation 

 toward ensuring the practically undiminished size of the ovum 

 at the end of the process; that is, an equal subdivision of the 

 chromatic elements is accompanied by an unequal subdivision 

 of the cytoplasm and deutoplasm. The general formation of 

 the large primary oocyte has been described. We should 

 emphasize again the fact that the maturation of this cell 

 frequently is not completed until after the sperm cell has 

 actually entered its substance. If we were describing the events 

 of the maturation of the egg in strict accordance with their 

 usual, though not invariable, time relations we should next 

 describe the ensemination of the egg the first step in fertiliza- 

 tion. For the sake of clearness, however, we shall describe 

 maturation as if it occurred before the entrance of the sperm; 

 as a matter of fact, there are a few forms in which this is really 

 the normal course of events, as in the sea-urchin and most 

 Echinoderms. 



The nucleus of the oogonium is very large and lies toward one 

 side of the cell practically always toward the animal pole of the 

 egg (Fig. 76, A). The first steps in the maturation of the ovum 

 closely resemble those in the sperm. During the brief synizesis 

 stage the chromatin condenses near the centrosome, closely 

 around the large "nucleolus" which is commonly found in most 

 oocytes; the emerging spireme shows that synapsis has occurred 

 for the spireme is segmented into the haploid number of ele- 

 ments, representing the bivalent chromosomes. That is, the 

 actual reduction occurs in the primary oocyte as in the primary 

 spermatocyte. The oocyte nucleus then passes through a 

 condition not represented in the spermatocyte in that a large 

 amount of chromatin leaves the chromosomes (spireme seg- 

 ments), either dissolving in the nuclear sap, or passing in the 

 form of granules or small masses to some region of the nucleus 



