STRUCTURE AND PHYSIOLOGY OF ANIMALS 



21 



completely separated off from the ovum as a rounded particle 

 the first 'polar body (pol.). A second division of the nucleus 

 results in the throwing off of a second polar body ; and. after this 

 has been formed, the portion which remains in the ovum resumes 

 its central position and forms what is termed the female pro- 

 nucleus (B, ^ pron.). The essential ultimate result of maturation 

 is the reduction of the number of chromosomes in the ovum by 

 one-half. 



In the process of impregnation a very minute body, the male 

 cell, sperm-cell, or sperm, penetrates into the interior of the female 

 cell or ovum, and the nucleus which it contains the male pro- 

 nucleus (C, $ pron.} coalesces with the female pronucleus to form a 

 single nucleus called the segmentation nucleus (E, seg. nucl.). 1 The 

 principal part in the process of fertilisation is thus played by the 

 two nuclei. The female centrosome 

 disappears : a male centrosome enters 

 with the sperm. 



Apparently in this process of 

 fertilisation some attraction is 

 operative between the male and 

 female cells. In many instances a 

 prominence (the receptive promin- 

 ence) is pushed out by the ovum 

 at the point where the sperm 

 enters. The female pronucleus, 

 leaving its former central position, 

 approaches the male cell as it enters. 

 In most cases a single sperm alone 

 enters the ovum in impregnation. 

 According to the older observers, 

 as soon as a sperm enters the 

 ovum, a membrane is formed 

 around the latter hindering the penetration of additional sperms ; 

 but it has now been shown that such a membrane occurs 

 only in certain cases, and is quite exceptional. That, as a general 

 rule, only one sperm penetrates into the ovum appears to be 

 due to the circumstance that, as a result of the entry 

 of the one sperm, the peculiar attraction above referred 

 to becomes in some way destroyed or diminished. But, though 

 the entry of one sperm only is usual, cases of the entry of several 

 -polyspermy, as it is termed are by no means rare, and would 

 appear to be quite normal in some groups of animals. 



In some animals the ovum develops parthenogenetically i.e. 

 without any process of fertilisation by means of a male cell. This 



1 In this coalescence the chromosomes of the male and female pronuclei do 

 not become fused, but remain quite separate and retain their distinctness 

 during the nuclear divisions that follow. 



FIG. G. Ovum of a Sea-Uretun, show- 

 ing the radially striated cell-mem- 

 brane, the protoplasm, containing 

 yolk-granules, the large nucleus (ger- 

 minal vesicle), with its network of 

 chromatin and a large nucleolus 

 (germinal spot). (From Balfour'a 

 Embryology, after Hertwig.) 



