76 CYTOLOGY CHAP. 



grounded on such experiments as those of Boveri (1907), who found that 

 sea-urchins' eggs into which, from pathological causes, two spermatozoa 

 had entered possessed four centrosomes, with a four-poled first cleavage 

 mitosis (tetraster) ; if three spermatozoa had entered the egg, six centro- 

 somes were found to be present. 



The important part played by the centrosome of the spermatozoon 

 introduces us to the thoroughly well established view that fertilization 

 has two functions (i) the stimulation of the egg to develop, and (2) the 

 fusion of the gamete nuclei. These two processes are not necessarily 

 interdependent. Thus an egg may be stimulated to development by 

 the entry of a spermatozoon which belongs to such a distantly related 

 species that nuclear fusion between them is impossible (p. 160), or the 

 stimulus may even be provided by physical means without the inter- 

 vention of a spermatozoon at all (artificial parthenogenesis, p. 94). 



Since an egg under certain circumstances may develop without fusion 

 of its nucleus with a microgamete nucleus, it is certain that karyogamy 

 is not a necessary condition of development. The function of the fusion 

 of the gamete nuclei must be looked for in its ulterior effects on variation 

 and heredity. The incapacity of either gamete to develop, under ordinary 

 circumstances, by itself, has indeed been looked upon as an adaptation 

 to ensure that karyogamy shall take place, the loss of power to develop 

 independently being attained in the case of the spermatozoon by its 

 general specialization and the reduction of its cytoplasm to a minimal 

 quantity, and in the case of the egg by the degeneration of the centro- 

 somes and rest of the achromatic figure after the completion of the meiotic 

 divisions. It is indeed difficult to believe that the developmental stimulus 

 given by the spermatozoon is not connected with the introduction of the 

 active male centrosome. It must however be remembered that in the 

 case of artificial parthenogenesis no new centrosome is introduced from 

 without. Moreover, in many eggs the pause in the meiotic processes in 

 which the egg awaits fertilization takes place in metaphase I. or II., that 

 is to say, at a time when its centrosomes and achromatic figure are fully 

 developed and active. Thus here, as in so many other biological problems, 

 a simple, almost mechanical, explanation is found to be inadequate to 

 cover all the facts. 



As a rule, only one spermatozoon enters the egg. Even when, as in 

 the great majority of cases, the egg is surrounded by an enormous number 

 of spermatozoa seeking to enter it, the penetration of a single spermatozoon 

 usually immediately confers on the egg the power of resisting the entrance 

 of any more. The means by which this resistance is effected is not fully 

 understood, though in many eggs the exclusion of supernumerary 

 spermatozoa is certainly aided by the secretion of a membrane 



