ZONA PELLUCIDA 



830 DEVELOPMENT [GIL LTX. 



the mature ovum and the two polar bodies contains half the 

 number of chromosomes present in the parent germinal cell. In 

 some cases the first polar body divides at the same time that the 

 second polar body is formed, and the process may be represented by 

 the schema in fig. 621. 



The nucleus of the mature ovum is known as the female pro- 

 nucleus. 



Impregnation. 



By impregnation is meant the union of a spermatozoon with an 

 ovum. The spermatozoon, moving by the flagellar movement of 



its tail meets the mature ovum 

 in the upper part of the Fallopian 

 tube, and by means of its sharp 

 head cap it pierces the zona pel- 

 lucida, and the head, neck, and 

 possibly part of the body, enter 

 the substance of the ovum, 

 where they undergo transforma- 

 tion, and are converted into a 

 male pronucleus with an attendant 

 attraction sphere and its centro- 

 ^, , ^ some. The male pronucleus con- 



Fio. 622. The fertilised ovum or blastosphere, 



showing its new nucleus and attraction tains the same number ot chromo- 



somes as the female pronucleus, 



for the mitosis which occur when the spermatocyst of the first order 

 divides to form the two spermatocysts of the second order, is a 

 heterotype mitosis, in which only half the usual number of chromo- 

 somes appear; and consequently the spermatocysts of the second 

 order, and their descendants the spermatids, also contain only half 

 the typical number of chromosomes. These are retained in the 

 spermatozoa, which are produced by modification of the spermatic!, 

 and they reappear in the male pronucleus. 



After the male pronucleus has formed in the substance of the 

 mature ovum, it approaches the female pronucleus, and when the 

 two pronuclei fuse, fertilisation is completed. The nucleus which 

 results from the fusion the first segmentation nucleus contains 

 the typical number of chromosomes, half being derived from the female 

 and half from the male germinal element. When the fertilisation is 

 completed, the segmentation nucleus is accompanied by two attrac- 

 tion spheres with their centrosomes (see fig. 622) ; one of these spheres 

 is that associated with the male pronucleus, but the origin of the other 

 is uncertain. It may belong to the ovum, though it was not apparent 

 during the maturation, or it may have been produced by the division 

 of the centrosome and attraction sphere which accompany the male 

 pronucleus. 



