66 



MALE PRONUCLEUS. 



FIG. 33. OVUM OF ASTERIAS GLA- 

 CIALIS, WITH MALE AND FEMALE PEO- 

 NDCLEtJS AND A RADIAL STRIATION OF 

 THE PEOTOPLASM ABOUND THE FOBJIEE. 

 (Copied from Fol.) 



entrance of other spermatozoa. At the point where the spermatozoon 

 entered a crater-like opening is left in the membrane, through which 

 the metamorphosed tail of the spermatozoon may at first be seen 

 projecting (fig. 32). 



The head of the spermatozoon when in the egg forms a nucleus, 



for which the name male pronu- 

 cleus may be conveniently adopted. 

 It grows in size, probably by assimi- 

 lating material from the ovum, and 

 around it is formed a clear space free 

 from yolk-spherules. Shortly after 

 its formation the protoplasm in its 

 neighbourhood assumes a radiate ar- 

 rangement (fig. 33). At whatever 

 point of the egg the spermatozoon may 

 have entered, it gradually travels 

 towards the female pronucleus. The 

 latter, around which the protoplasm 

 no longer has a radiate arrangement, 

 remains motionless till the rays of 

 the male pronucleus come in contact 

 with it, after which its condition uf 

 repose is exchanged for one of activity, 

 and it rapidly approaches the male pronucleus, apparently by means 

 of its inherent amoeboid contractions, and eventually fuses with it (figs. 

 3436). 



As the male pronucleus approaches the female the latter, according 

 to Seleuka, sends out protoplasmic processes which embrace the former. 

 The actual fusion does not take place till after the pronuclei have 

 been in contact for some time. While the two pronuclei are 

 approaching one another the protoplasm of the egg exhibits amoeboid 

 movements. 



The product of the fusion of the two pronuclei forms the first 

 segmentation nucleus (fig. 37), which soon, however, divides into the 

 two nuclei of the two first segmentation spheres. 



The phenomenon which has just been described consists essentially 

 in the fusion of the male cell and the female cell. In this act the 

 protoplasm of the two cells as well as their nuclei coalesce, since the 

 whole spermatozoon which has been absorbed into the ovum is a cell 

 of which the head is the nucleus. 



It is clear that the ovum after fertilization is an entirely different 

 body to the ovum prior to that act, and unless the use of the same 

 term for the two conditions of the ovum had become very familiar, a 

 special term, such as oosperm, for the ovum after its fusion with the 

 spermatozoon, would be very convenient. 



Of the earlier observations on tins subject there need pei-liaps only be 

 cited one of E. van Beneden, on the rabbit's ovum, shewing the presence of 



