IMPREGNATION OF THE OVUM. 51 



short distance into the interior. It is uncertain whether the whole tail of the 

 spermatozoon enters the ovum or not. In some of the lower vertebrates and in 

 other animals, it appears to do so. It is probably always the case-that at least 

 the main piece of the tail enters the yolk. The tail as such very soon disappears, 

 while the head of the spermatozoon enlarges, probably by the imbibition of 

 fluid from the surrounding yolk. The head of the spermatozoon is rich in chro- 

 matin, which forms a series of irregujar masses as the head enlarges, producing 

 a network appearance, and is thus converted into a nucleus-like body, the 

 male pro-nucleus. At the same time the growing head surrounds itself in some 

 animals by a membrane. 



We now have a cell which contains two nucleus-like bodies, one derived 

 from the head of the spermatozoon and the other from the nucleus of the egg- 

 cell. They are termed respectively the male and female pro-nucleus. Each 

 pro-nucleus, when it first appears, is small and gradually enlarges, probably in 

 both cases by the imbibition of fluid. The relative size of the two pro-nuclei 

 varies considerably in different species, and is probably a secondary and rela- 

 tively unimportant relation. The proportion between the two probably depends 

 upon the time when the male pro-nucleus is formed. If the spermatozoon en- 

 ters early while the female pro-nucleus is forming, it may make a pro-nucleus as 

 large as that from the egg-cell. If, on the other hand, the spermatozoon enters 

 late, the female pro-nucleus enlarges, acquires a start, and the growing male pro- 

 nucleus is, therefore, smaller. 



Concerning the fate of the middle piece of the spermatozoon and its share 

 in the fertilization in the ovum of mammals, we possess no satisfactory informa- 

 tion. It has been shown, however, in other animals that this middle piece pro- 

 duces a centrosome, and the only centrosome which appears in the fertilized 

 ovum. The theory has been advanced that the ovum, after its maturation, has 

 no centrosome, that a centrosome is always brought into the ovum by the sper- 

 matozoon in the manner just indicated. If we regard the centrosome as a per- 

 manent cell element, then we must further interpret the addition of the male 

 centrosome as one of the most important phenomena of fertilization. Whether 

 this hypothesis is correct or not, we are unable at present to decide. 



Astral figures play a conspicuous part in the phenomenon of fertilization in 

 many animals. Astral figures are produced in the protoplasm of the ovum by 

 its assuming a special radiating structure. Astral figures may appear around 

 both the male and female pro-nuclei (Fig. 3). In other cases the astral figure 

 arises only in association with the head of the spermatozoon or male pro-nucleus. 

 In mammals, so far as known, no astral figures are developed about either of the 

 pro-nuclei. There is a clear space in the protoplasm around each nucleus, and 



