20 



HUMAN EMBKYOLOGY. 



FERTILISATION. 



Fertilisation is the term applied to the union of the male with the female gamete 

 to form a zygote which contains the typical number of chromosomes (Fig. 23). 



The meeting of the gametes and their union take place, normally, in the upper 

 or middle part of the uterine tube. 



The details of the process are unknown in the case of the human subject, but 

 in many animals it has been noted that as the spermatozoon approaches the ovum 

 the latter shows signs of excitement, and a small prominence, called the cone of 

 attraction, appears on its surface. At the same time its pronucleus undergoes 

 changes of form. As the two gametes meet the spermatozoon pierces the oolemma 

 which surrounds the ovum and passes through the cone of attraction into the body 

 of the ovum. 



In some cases apparently only the head, neck, and body of the spermatozoon 

 effect an entrance, but in others the whole spermatozoon enters the body of the ovum. 



After the entrance occurs and before the second polar body is formed, the parts of 



Spermatozoon 

 Parts of first polar body X 



Second 

 polar body 



Polar bodies 



Centrosome with 

 male pronucleus 



;Male pronucleus 



^.Female 

 pronucleus 



Oolemma 



jSC,Body of 



/ mature ovum 



Polar bodies 



Polar bodies 



Male pronucleus^ 



Centrosome with ^ 

 male pronucleus -j 



pronucleus 



Oolemma 

 Centrosome 



First 



segmenta- 

 tion nucleus 



-rf - Centrosome 



FIG. 23. SCHEMA OF THE FERTILISATION OF THE MATURE OVUM AND THE FORMATION OF THE ZYGOTE. 



the spermatozoon which have entered remain quiescent. After the second polar 

 body is formed they disappear and are replaced by a nucleus which contains half the 

 typical number of chromosomes and which is accompanied by two centrosomes. At 

 this period the impregnated ovum contains two pronuclei, both of which contain 

 half the typical number of chromosomes ; but the female pronucleus has no accom- 

 panying centrosomes. 



Shortly after the appearance of the male pronucleus the two pronuclei unite 

 and then the zygote, formed by the union of the male and female gametes, 

 consists of a cell body enclosing a nucleus called the first segmentation nucleus ; and 

 two centrosomes. 



The first segmentation nucleus is the product of the union of the male and the 

 female pronuclei. It contains the typical number of chromosomes, half being 

 derived from the male and half from the female gamete ; and it is accompanied by 

 two centrosomes, both of which appear to be derived from the male gamete, though 

 their exact origin has not yet been definitely established. The zygote and the 

 polar bodies which are still present are enclosed within the oolemma. 



