136 Heredity and Enmronment 



egg does not begin to develop until a spermatozoon has entered it. 



b. Union of Germplasms. But the spermatozoon not only stim- 

 ulates the egg to develop, as environmental conditions may also 

 do, but it carries into the egg living substances which are of great 

 significance in heredity. Usually only the head of the spermato- 

 zoon enters the egg (Fig. 4) and this consists almost entirely of 

 nuclear chromatin (Fig. 4 D-H, 42 A-B) ; when the egg has ma- 

 tured and is ready to be fertilized its nucleus also consists of a 

 small mass of chromatin (Fig. 42 C). Both of these condensed 

 chromatic nuclei then grow in size and become less chromatic by 

 absorbing from the egg a substance which is not easily stained 

 by dyes and hence is called achromatin (Figs. 4 I-L, 42 D-E). 

 The chromatin then appears to become scattered through each 

 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. 4 F-7, 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 



