136 Heredity and Environment 



nucleus in the form of granules or threads which are embedded in 

 the achromatin ; this is the condition of a typical "resting" nu- 

 cleus. It is evident however that these chromatin granules are 

 not scattered broadcast throughout the nucleus, since at the next 

 mitosis they come together into particular chromosomes similar in 

 every way to the chromosomes of the previous mitosis. Probably 

 the chromosomes preserve their identity from one division to the 

 next either in the form of chromosomal vesicles (Fig. 8, p. 20) 

 or as strings of granules. The spermatozoon also brings into the 

 egg a centrosome or division center, around which an aster ap- 

 pears consisting of radiating lines in the protoplasm of the egg 

 (Fig. 4F-I, Fig. 42, B-E). 



The moment that the spermatozoon touches the surface of the 

 egg the latter throws out at the point touched a prominence, or 

 reception cone (Fig. 4 A-E), and as soon as the head of the 

 sperm has entered this cone some of the superficial protoplasm of 

 the egg flows to this point and then turns into the interior of the 

 egg in a kind of vortex current. Probably as a result of this 

 current the sperm nucleus and centrosome are carried deeper into 

 the egg and finally are brought near to the egg nucleus (Fig. 42, 

 D and E). In the movements of egg and sperm nuclei toward 

 each other it is probable that they are passively carried about by 

 currents in the cytoplasm ; the entrance of the sperm serves as a 

 stimulus to the egg cytoplasm which moves accordingly to its pre- 

 established organization. 



2. Cleavage and Differentiation. — When the sperm nucleus 

 has come close to the egg nucleus the sperm centrosome usually 

 divides into two minute granules, the daughter centrosomes, which 



Sp.). C, Second maturation division (2d Mat. Sp.) and first polar body 

 (1st PB) resulting from first division. $N, Sperm nucleus. $ C, Sperm 

 centrosome. D, Approach of sperm nucleus ( $ N) and sphere (SS) to 

 egg nucleus ( ? N) and sphere ( 2S) ; the second polar body (2d PB) has 

 been formed and the first has divided (1st PB). E, Meeting of egg and 

 sperm nuclei and origin of cleavage centrosomes. F, First cleavage of 

 egg showing direction of currents in the cell. 



