1 82 Experimental Zoology 



by Loeb. If to loo parts of an artificial sea water containing 

 0.3 cubic centimeter sodium hydroxide the eggs of Strongylo- 

 centrotus and the sperm of the starfish be added, as many as 50 

 to 80 per cent of the eggs will be fertihzed. The eggs segment 

 and it appears that even the early stage of development — more 

 or less abnormal — may be reached. 



Polyspermy 



In the great majority of known cases only a single spermatozoon 

 normally enters the egg to fertilize it. The moment one sperma- 

 tozoon penetrates, some reaction takes place in the egg that pre- 

 vents other spermatozoa from entering. The reaction involves, 

 in some cases, contraction of the egg from the surrounding mem- 

 brane ; in other cases the formation of a membrane ; and we may 

 infer that other kinds of changes in the protoplasm may in other 

 cases interfere or prevent the entrance of more than one sperma- 

 tozoon. If the egg is injured or narcotized or is stale, more than 

 one spermatozoon may enter. The result is generally injurious^ 

 for, while a regular or more often an irregular cleavage may fol- 

 low, the embryo fails to develop properly. This failure seems 

 to be due, in the main, to the unequal distribution of the chroma- 

 tin or other materials to the different parts of the egg. 



It has been found in hybridizing that in certain cases more 

 than a single sperm enters the ^gg^ and the irregularities in 

 cleavage that follow seem to be due in part to this condition, but 

 in other cases where one spermatozoon enters, the failure to 

 develop must be ascribed to different causes. In these cases 

 the results may be due sometimes to irregularities in the mechan- 

 ism of division, but at other times to some incompatibihty or 

 failure of the two uniting elements to work together to a com- 

 mon end. 



According to the degree of perfection to which the distribution 

 of the materials of the egg is carried out, the subsequent develop- 

 ment is more or less perfect. Even in cases where the hybrid 

 reaches the adult condition, it has been observed that it may have 

 a weak constitution, and even when it is strong the hybrid is 



