THE GERM CELLS. 9 



woman, while a single ejaculation of semen may contain two hundred 

 million spermatozoa. 



Significance of Germ Cell Organization. One feature of this has already 

 been mentioned in connection with the morphological differences between the 

 male and female germ cells: The spermatozoon is adapted for locomotion 

 while the ovum is passive and frequently laden with yolk. This diversity in 

 structure is truly correlated with a physiological division of labor. The two 

 cells must unite before development of a new organism can proceed; the egg 

 is non-motile and contains nutriment for the future embryo, the sperm by 

 virtue of its motility approaches the egg and finally enters it. 



Another feature of organization is embodied in the chromatin. The 

 chromatin is a visible substance and is regarded as the inheritance material. 

 Its constitution is such that it determines in large measure the course of 

 development of the embryo arising from the united germ cells and the quali- 

 ties or characters of the adult. Parts of the chromatin contain or comprise 

 factors which give rise to certain characters in the developed organism. 

 These factors, or genes as 'they are frequently called by students of heredity, 

 are not visible things but are probably expressed in the physico-chemical 

 nature of the chromatin. There is ample evidence for their presence, upon 

 which is based the modern theory of heredity or Mendelian inheritance. One 

 set of factors is present in the ovum and another in the sperm. Their rela- 

 tion to the chromosomes and their behavior will be considered in the two 

 succeeding chapters. 



There are certain characters of the embryo that are derived directly from 

 the cytoplasm of the ovum ; so chromatin is not the only germ cell substance 

 that influences development. Since these characters come from the female 

 parent and not from the male, this is sometimes called maternal inheritance 

 as distinguished from Mendelian inheritance. The cytoplasm of the sperm 

 seems to be useful only as a temporary locomotor apparatus. The egg cyto- 

 plasm is so organized that it becomes potent in determining the course of 

 development. In the case of an ovum that contains a moderate amount of 

 yolk, as in the frog, or a large quantity, as in the bird, there is an obvious 

 polar differentiation or polarity which is visibly expressed in the distribution 

 of the cytoplasm and yolk. This polarity of the egg determines the polarity 

 of the future adult animal. It will be seen in a later chapter that the egg of 

 Amphioxus is bilaterally symmetrical, and that the bilateral character of 

 the developing animal follows upon that of the egg. This is true also of 

 the frogs and fishes. Other evidence of the internal organization of the 

 egg cytoplasm in certain invertebrates is seen in collections of various pig- 

 ments in the ova; and it is possible to predict accurately the part of the em- 

 bryo that will be derived from the portion of the cytoplasm containing a given 



