40 SEX 



inheritance of the offspring, the spermato- 

 zoon also introduces a minute body known 

 as the centrosome, which plays an important 

 part in the subsequent division of the fertilised 

 ovum. (4) It is also certain that the sperma- 

 tozoon acts as a stimulus which sets the egg 

 dividing or, it may be, removes some in- 

 hibiting factor which was keeping the egg 

 from dividing. In a variety of cases what 

 the sperm does, whether in stimulation or in 

 removing a check, can be replaced artificially, 

 and this in many different ways. The experi- 

 ments of Loeb, Delage, and others have shown 

 that the ova of starfish, sea-urchin, worm, 

 mollusc, fish, and even frog can be made to 

 divide without being fertilised. Alterations in 

 the chemical and physical state of the medium, 

 e. g. the addition of a little magnesium chloride 

 to the sea-water, may replace ordinary fertilisa- 

 tion, at least in so far as setting division agoing 

 is concerned. The artificial stimuli effectively 

 used are very varied physical, chemical, and 

 mechanical an electric shock, a superabun- 

 dance of carbonic acid gas, and even a pin- 

 prick. They differ for different kinds of eggs, 

 and even for eggs of the same kind at different 

 stages of ripeness. Perhaps there is some 

 common factor in all the effective stimuli, 

 but what it is remains unknown. According 

 to some experimenters, the egg responds 

 directly to the external stimulus; according 

 to others the artificial conditions do not 

 directly induce segmentation, but modify the 

 intimate condition of the egg in such a way 

 that it becomes auto-parthenogenetic. The 

 second view is suggested by cases in which the 



