THE FROG: HISTOLOGY, GERM CELLS, DEATH 109 



mitosis, but any difference that may exist between the 

 chromosomes will cause the germs to which they are bodily 

 transferred to be unlike. 



Only one spermatozoon ever conjugates with any ovum. 

 While the slime around the eggs is swelling up and setting 

 to a jelly in the water, the spermatozoa which have been 

 shed over it by the male (p. 67) pass through it, swimming by 

 means of their tails. They are far more num- 

 erous than the ova and most of them perish, 

 but one succeeds in entering each egg. Thus a zygote 

 known as the oosperm or fertilised ovum comes into being. 

 The cytoplasm of the spermatozoon disappears in that of 

 the ovum, but the nucleus passes onward and comes to lie 

 side by side with that of the egg. The two nuclei are known 

 as the male and female pronuclei. Meanwhile there has 

 arisen from the neck of the spermatozoon a centrosome, 

 around which is formed an aster. As the nuclei approach 

 one another this divides and forms a spindle. The pro- 

 nuclei break up each into twelve chromosomes, which lie 

 at the equator of the spindle. Thus the normal number 

 of twenty-four chromosomes is restored; these lie in two 

 groups corresponding to the two pronuclei. Now the 

 chromosomes split in the ordinary way and the halves 

 pass to opposite poles of the spindle, where they form 

 ordinary nuclei. The cytoplasm of the egg meanwhile 

 divides into two cells, known as the first two blasto- 

 meres. This is the first of the series of divisions known 

 as the cleavage or segmentation of the ovum, by which the 

 cells of the embryo are formed (see p. 487). 



The frog which is developed from trie fertilised ovum 

 h m w '^' ^e an y otner amma l> inherit the like- 



ness of its parents: large frogs will have large 

 offspring, and so forth. This, of course, because the 

 embryo is physically continuous with each parent through 

 one of the germs. The question suggests itself: is any 

 constituent of the germ specially responsible for heredity ? 

 Since the nucleus is known to exercise a directive influence 

 over many of the activities of the cytoplasm, and again 

 since the spermatozoon contributes hardly any cytoplasm 

 to the embryo, it is natural to assume that the principal 

 though perhaps not the only agent of heredity is the nucleus. 



