960 



MATURATION OF THE OVUM. 



radiate form (diaster). When this has taken place, the peripheral pole 

 of the nucleus of the ovum thus altered appears above the surface of 

 the ovum, becomes constricted off and expelled from the ovum like 

 a waste product in the form of a small body (Fig. 358, VI and VII). 

 The formation of a second polar body takes place again by mitosis in 

 the same way during the penetration of the spermatozoon. Both of 

 the bodies thus eliminated, which are of no further use in the growth 

 and development of the ovum, are designated directing bodies or polar 

 cells. (Figs. 369 and 370.) The remaining portion of the germinal 

 vesicle lying near the center remains within the yolk, wanders back 

 toward the center of the ovum, increases in size, and thus forms the egg- 

 nucleus or female pronucleus, which has no centrosome. 



The spermatozoon that has entered the ovum moves toward the female 

 pronucleus, its head becoming surrounded by a radiating crown; then 



Nucleus and 

 radially arranged ^ 

 protoplasm. &*- 



First division. 



, Segmentation spheres. Morula. 



FIG. 371. Four Stages of Division of an Impregnated Ovum of Echinus saxatilis. 



its cilmm is dissolved and its head, alone remaining, forms a chromatic 

 mass and swells into a second new nucleus, the sperm-nucleus or the 

 male pronucleus. From the connecting segment a centrosome (sur- 

 rounded by rays) develops and this soon becomes directed toward the 

 Qterior of the ovum. The centrosome of the male pronucleus also 

 livides. I he male and female pronuclei now unite to form the new 

 nucleus of the impregnated ovum, with which both of the sperma- 

 spneres resulting from the division are in contact; and the yolk 

 assumes a radiating appearance (Figs. 370 and 371). 



The entrance of several spermatozoa into the ovum (polyspermism) takes 

 place normally in large ova rich in yolk. From these accessory sperm-nuclei 

 develop all of which probably disappear later. O. Hertwig and Fol made the 

 remarkable observation (m echinoderms) that several embryos form from one 

 ovum, when several spermatozoa enter the ovum abnormally. The male pro- 

 nuclei resulting from the individual spermatosomes then each unite with a frag- 

 ment of the disintegrated female pronucleus. 



