216 PHYSICAL BASIS OF HEEEDITY 



StrongylocentrotuSj are fertilized by the sperm of another 

 sea urchin, Sphaer echinus, the segmentation nucleus, 

 formed by the union of the egg- and sperm-nucleus shows 

 irregularities in the movements of the daughter chromo- 

 somes to the poles of the spindle. While some of the 

 chromosomes after dividing pass normally to the poles, 

 others become scattered irregularly between the two poles 

 and fail to become incorporated in the two-daughter nuclei 

 (Fig. 99, a). They appear to become lost and take no 



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Fig. 100. — Fertilization of an egg that had started to develop parthenogenetically. 

 The belated sperm unites with one of the daughter chromosomes groups only, o; an 

 earlier condition of the same prosedure. (After Herbst.) 



part in the further development. Counts of the chromo- 

 some plates in the later divisions of the egg give about 

 21 chromosomes, whereas 36 are expected as the whole 

 number. It appears that 15 chromosomes are lost, and 

 presumably they belong to the foreign sperm. Many of 

 these eggs develop abnormally, but those that reach the 

 pluteus stage show a maternal skeleton only. This seems 

 to mean that the sperm has done no more than start the 

 development. It has contributed nothing, or little, to the 

 embryo, and it seems reasonable to attribute this to the 



