REPRODUCTION. 63 



of the male cell. Once more we are led to see the importance 

 of this structure in the life of the cell. 



Fertilization of the Ovum. — The spermatozoon, lashing its 

 way along, when it meets the ovum, enters it either through a 

 special minute gateway (micropyle), or, if this be not present- 

 as it is not in the ova of all animals — actually penetrates the 

 membranes and substance of the female cell, and continues act- 

 ive till the female pronucleus is reached, when the head enters 

 and the tail is absorbed or blends with the female cell. The nu- 

 cleus of the male cell prior to union with the nucleus of the 



F.PNt 



F.PNr 



-M.PN. 



Fig. 6;i.— Fertilization of oviim of a molluEik {Elysia mridis\ A, Ovum Bending up a 

 protuberance to meet the epermatozoOn. B. Approach of male pronucleus to 

 meet the female pronucleus. F. PN^ female pronucleus; M. PN, male pronucleus, 

 S, spermatozoon. 



ovum un^rgoes changes similar to those that the nucleus of the 

 ovum underwent, and thus becomes fitted for its special func- 

 tions as a fertilizer ; or perhaps it would be more correct to say 

 that these altered masses of nuclear substance mutually fertil- 

 ize each other, or initiate changes the one in the other which 

 conjointly result in the subsequent stages of the development 

 of the ovum. The altered male nucleus (male pronucleus), on 

 reaching the female pronucleus, finds it somewhat amseboid, 

 a condition which may be shared in some degree by the entire 

 ovum. The resulting union gives rise to the new nucleus (seg- 

 mentation nucleus), which is to control the future destinies of 

 the cell ; while the cell itself, the fertilized ovum (oosperm), en- 

 ters upon new and marvelous changes. 



In reality this process was foreshadowed in the dim past of 

 the history of living things by the conjugation of infusoria 

 and kindred animal and vegetable forms. When lower forms 

 (unicellular) conjugate they become somewhat amoeboid sooner 

 or later, and division of cell contents results. In some cases 

 (septic monads) the resulting cell may burst and give rise to a 



