FERTILIZATION 255 



tributes to the formation of the offspring a single cell, in which 

 must be sought the solution of the problems of sex, fertilization, 

 development, and inheritance. However, the concentration of at- 

 tention on the cell has not simplified the solution of these funda- 

 mental problems; but rather it has contributed to an ever- 

 increasing appreciation of the complexities of cell phenomena and 

 the difficulties of formulating them in general terms. (Fig. 303.) 



2. Significance of Fertilization 



Quite naturally the original view was that fertilization funda- 

 mentally is reproduction — the mature egg pauses in develop- 

 ment and usually comes to naught unless a sperm enters. How- 

 ever, as we know, reproduction is cell division or the detachment 

 of a portion of a living organism to form another, whereas fertiliza- 

 tion is the union of two cells to form one cell. The erroneous idea 

 that fertilization is reproduction is due to the fact that in higher 

 organisms, if fertilization is to occur at all, it must take place at 

 the period in the life history when the individual is but a single 

 cell detached from the parent — that is, at reproduction. With 

 this point clear, we may briefly discuss the significance of ferti- 

 lization, first on the basis of evidence derived from the Protozoa. 



Protozoa. The life histories of nearly all Protozoa that have 

 been carefully studied include a period in which fertilization occurs. 

 Under favorable environmental conditions, Paramecium, for in- 

 stance, reproduces by binary fission two or three times a day so that 

 in a remarkably short period the one cell is replaced by a host of de- 

 scendants. Sooner or later, however, the individuals exhibit a 

 tendency to unite temporarily in pairs, or conjugate. During 

 conjugation complicated changes take place in the nuclei of the 

 cells, involving chromosome reduction and the formation of two 

 gametic nuclei in each individual of the pair of conjugants. Then 

 one of the gametic nuclei in each conjugant migrates over and 

 fuses with the stationary gametic nucleus of the other to form 

 a synkaryon, or fertilization nucleus, in each cell. After this the 

 two Paramecia separate, reconstruct their characteristic vegeta- 

 tive nuclear apparatus, and proceed to reproduce by division as 

 before. (Fig. 170.) 



This is fertilization in Paramecium, and on the assumption 

 that the primary significance of synkaryon formation should be 

 most evident in unicellular forms, of which, of course, this animal 



