POLYSPERMY IO5 



between physical and chemical mechanisms in biological systems, 

 except in the case of such phenomena as electrotonus, is of ques- 

 tionable value; but for what this distinction is worth, Ziegler 

 (1898), who also did constriction experiments with cotton threads 

 on sea-urchin eggs, found that if he constricted the fertilized egg 

 in such a way that the spermatozoon was in one part, connected by 

 a narrow bridge of cytoplasm to the rest of the egg, which contained 

 the female nucleus, that part of the egg which contained the female 

 nucleus failed to divide though the nucleus showed signs of 

 activity, while the part containing the sperm head did divide. A 

 somewhat similar situation occurs in the eggs of Crepidula plana 

 Say, in which only that part of the egg which contains the sperm 

 nucleus divides, the other part being called, rather tautologically, 

 a polar body. Evidently one of the essential features in fertiliza- 

 tion is the introduction of a division centre into the egg by the 

 spermatozoon, 'rather than a diffuse chemical action of the sperm' 

 (T. H. Morgan, 1927, p. 512). In recent years little work has been 

 done on the problems raised by Type II Inhibition of polyspermic 

 development. Further experiments on newt eggs, which are easily 

 obtained, would bring their own reward. 



Failure of the Inhibition. Type I and Type II Inhibition may 

 fail or be induced to fail, and an immense amount of work was done 

 on the consequences of such failures in the nineteenth and early 

 twentieth centuries (see, for example, Boveri, 1907). When more 

 than one sperm nucleus engages in syngamy, abnormal cleavages 

 occur, followed by the early death of the pathological embryo. 

 Some cases of dispermic adults are, however, known in insects 

 (Goldschmidt & Katsuki, 193 1 ; L. V. Morgan, 1929), and in birds 

 (Hollander, 1949), in which large patches of feathers, coloured 

 differently from the rest of the animal, are found on occasions. In- 

 teresting accounts of two 'mosaic' cocks, as such animals are called, 

 bred from the same sex-linked cross. Light Sussex $ X Rhode 

 Island 3, and of a mosaic daughter of a mosaic cock, will be found 

 in papers by Greenwood & Blyth (195 1) and Blyth (1954). These 

 mosaics may sometimes be caused by dispermic fertilization, in 

 which case part of the tissue of the animal has a set of genes derived 

 from one spermatozoon and part from a different set of genes 

 derived from the other spermatozoon. The evidence for the ex- 

 istence of dispermic adult humans, or indeed any mammals, is not 

 conclusive. 



