THE PHYSICAL BASIS OF HEREDITY. 173 



spermatozoon in some cases containing less than t * 6ir 

 (Wilson), and in extreme cases less than T^nn^J-oouo 

 (Hertwig) of the volume of the egg cell. 



A discussion of the method by which the reduction 

 of the chromosomes in the germ nuclei is brought about 

 may profitably be deferred until the es- 

 Fertihzation. sential features of fertilization have been 

 examined. The phenomena of the fusion of egg and 

 sperm can best be studied in some such form as the sea 

 urchin, in which the egg is very small and, in some spe- 

 cies, quite transparent. As fertilization takes place free 

 in the sea water, the germinal cells being cast out from 

 the parents, it is possible to collect the eggs and sperm 

 separately from mature individuals and bring them to- 

 gether in small dishes of sea water, and at such times 

 as may suit one's convenience. Then in the living egg 

 much of the process may be followed under the micro- 

 scope, and properly prepared sections of the eggs killed 

 by reagents at the various stages enable conclusions to 

 be drawn as to matters of minute detail. Fig. 10, A to 

 f, presents a series of diagrams, taken from Boveri, illus- 

 trating the principal facts in the process of fertiliza- 

 tion. In Fig. 10, A, the egg is represented with its 

 clear nucleus in the centre, surrounded by the egg mem- 

 brane. Clustered around the periphery are a number of 

 spermatozoa endeavouring to find their way into the 

 substance of the egg. On the right-hand side in the 

 figure one has penetrated the membrane and is shown 

 passing into the egg cytoplasm, which puts forth a 

 small conical prominence to meet it. As soon as the 

 head of one sperm enters the egg cytoplasm a new 

 membrane is formed around the egg which effectually 

 prevents the entrance of any others. The head and 

 middle piece penetrate into the egg, the tail usually re- 

 maining imbedded in the membrane where it soon de- 



