132 GENERAL EMBRYOLOGY 



terms to include the whole history of the germ cells up to 

 the time of their formation as completely specialized struc- 

 tures, and maturation therefore becomes a phase in 06- and 

 spermatogenesis. 



It is a familiar fact that in fertilization the union of an egg 

 nucleus and a sperm nucleus is an essential step. The repeated 

 union of nuclei of the usual type, in this way, would result in 

 rapid and limitless increase in chromatic elements and material, 

 but for the operation of some mechanism preventing such an 

 accumulation, and yet permitting the fusion of germ-cell nuclei. 

 Maturation is such a process; but it is much more than this. 

 Consideration of all the phenomena of maturation raises many 

 questions, important, even fundamental, in their biological 

 significance. The full meaning of the phenomena can be 

 appreciated only in connection with the fertilization processes 

 to which they are introductory ; we may most profitably, there- 

 fore, postpone much of our discussion of the general significance 

 of maturation until we have become acquainted with the 

 process of fertilization. 



At this time, then, we shall describe the essential facts of 

 maturation as we find them in typical, in some respects perhaps 

 schematized, form, together with a brief comparative account 

 of some maturation processes in a few special instances. Then 

 after a similar account of fertilization in the next chapter, we 

 shall be in position to consider some of the general aspects of 

 both these processes taken together. The events of maturation 

 and fertilization are really closely related in time, as well as in 

 significance. While the spermatozoa are always fully mature 

 before they enter the egg cells, the entrance of the sperm may 

 occur either before, during, or after the maturation of the ovum, 

 although of course the essential step in fertilization, namely, the 

 union of the nuclei, does not occur (excepting in some Protozoa 

 and plants) until after the maturation of the egg nucleus is 

 completed, 



The maturation of the germ cells is accomplished, in the Meta- 

 zoa, by a modification of a mechanism common to all cells and 

 already familiar, namely, mitosis. But the cell divisions which 



