76 



EVOLUTION AND GENETICS 



Observation has reached, for the time being, its 



limit. 



Certain evidence relating to inheritance through 

 the sperm led to the conclusion that the chromosomes 

 are the bearers of the hereditary units. If so, there 

 should be many such units carried bv each chromo- 



v.-'ii i^ 





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Fig. 31. — Diagram to show stages in fertilization of an 

 egg by a spermatozoon. The three polar bodies lie at one 

 pole, and the spermatozoon is represented as entering near 

 the opi^osite side of the egg in 1 and 2. The head of the 

 sperm swells up and moves towards the egg-nucleus, that 

 has reformed after the polar bodies have been given off. 

 A centrosome forms near the sperm-nucleus. It divides 

 into two centrosomes, which begin to se])arate as a central 

 spindle appears between them. Around each centrosome 

 astral rays develop. The two nuclei come together in the 

 middle of the egg to become the segmentation nucleus. A 

 sjDindle develops around the nucleus. 



