GO 



THE CELL 



division with the usual longitudinal cleavage of its chromosomes and 

 the formation of two daughter nuclei. One of these and its centrosome 

 are extruded from the cell as the first polar body. 

 Ji The remaining nucleus and centrosome again divide 

 .^ M mitotically, only in this second division, instead of 

 \y)i the usual longitudinal cleavage of chromosomes, by 

 which each daughter nucleus is provided with the 

 / same number of chromosomes as the mother nucleus, 

 the chromosomes simply separate, one-Jialf going to 

 each daughter nucleus. One of the daughter nuclei 

 and its centrosome are now extruded as the second 

 polar body. The polar bodies ultimately disappear, 

 as does also the centrosome remaining within the 

 egg. This leaves in the now matured ovum a single 

 nucleus, which is known as the female pronucleus^ 

 and which contains one-half the number of chromo- 

 somes characteristic of cells of the species. 



During this process in some animals — in others 

 after its completion — the spermatozoon enters the 

 ovum. The head of the spermatozoon becomes 

 the male pronucleus, the middle piece becomes a 

 centrosome, while the tail is, in some instances at 

 least, left behind as the spermatozoon enters the egg. 

 The chromatin of the male pronucleus next becomes 

 arranged as chromosomes. Male and /emale pronuclei now lose 

 their limiting membranes and approach each other, their chromo- 

 somes intermingling. As each pronucleus contained one-half the 

 number, the monaster thus formed contains the full number of 

 chromosomes characteristic of the species. Meanwhile the male 

 centrosome, formed from the body of the spermatozoon, divides 

 into two daughter centrosomes. These with their radiating fibrils 

 have the same arrangement relative to the monaster of mingled 

 male and female chromosomes, already described under mitosis. 

 By longitudinal cleavage of these chromosomes, as in ordinary 

 mitosis, two sets of daughter chromosomes are formed. Each set 

 passes along the filaments of the achromatic spindle to its centro- 

 some. Thus is formed the diaster. By continuation of the mitotic 

 process two new nuclei are formed, each nucleus containing the 

 number of chromosomes characteristic of the species, and each being 

 made up equally of male and female chromosome elements. Thus 



.6 



Fig. II.— Hu- 

 man Spermatozoa. 

 (After Retzius.) 

 I, Head seen on 

 flat; 2, head seen 

 on edge; k, head; 

 m, body;/, tail; e, 

 end piece. 



