i INTRODUCTION 17 



Occasionally, it is true, the sex-chromosome remains distinct during the 

 resting period. We may imagine, by an act of faith, that the others 

 too retain their identity although we cannot see them, but it seems to 

 us that the only meaning which can be given to such an identity would 

 be the persistence of a centre for the synthesis of some special 

 sul istance. Lastly, it may be that in some cases sex is not irrevoc- 

 ably determined in the germ, but can be determined by feeding. 

 This at any rate has been asserted by Born (1881) in the case of 

 tadpoles though his results have not passed unquestioned. Wilson 

 attempts to get over this difficulty by supposing that the sex- 

 chromosome is only one of the factors which determine sex. 



Our final conclusion is that investigators have only touched the 

 fringe of an intensely interesting and important subject, and that a 

 great deal more research must be done before definite conclusions 

 can be arrived at. 



The meaning of the process of fertilization has proved a fascinating 

 subject for speculation. That the union of the two nuclei is not per 

 se necessary for development, is proved by the experiments of Loeb 

 and his pupils, who have caused the unfertilized eggs of Echinodermata, 

 Annelida, and Mollusca to go through the early stages of their 

 development by increasing the salinity or alkalinity of the water in 

 which they lie, i.e. by immersing the egg in what is called a hyper- 

 tonic solution, or by causing the egg to form a vitelline membrane 

 by rapid treatment with butyric and similar organic acids. By 

 uniting these two methods, i.e. by first causing the egg to form a 

 membrane through treating it with butyric acid and then treating it 

 with a hypertonic solution, a close approach to normal development 

 may be attained. 



The formation of the vitelline membrane is due, according 

 to Loeb, to incipient cytolysis, i.e. the peripheral protoplasm 

 breaks down, forming minute globules which cohere together so as to 

 form the membrane. Too long exposure to the acid causes the egg 

 to die, by a continuation of the same process until the whole cyto- 

 plasm is resolved into a mass of globules, but this process is arrested 

 by the action of the hypertonic solution. Therefore Loeb concludes 

 that the influence of the spermatozoon is primarily chemical ; in fact he 

 supposes that it carries into the egg a ferment, " lysin," which starts 

 the process of cytolysis, and also another substance which arrests 

 this process after it has resulted in the formation of a membrane. 



Loeb himself and his pupils Godlewski (1906) and Kupelwieser 

 (1906) have shown that it is possible to fertilize the eggs of a Sea- 

 urchin with the sperm of a Crinoid, a Star-fish, and even of a 

 Mollusc. In these cases the resulting organism, as long as it lives, 

 resembles exactly the normal larva which would have resulted if the 

 egg of the Sea-urchin had been fertilized by its own sperm, and 

 betrays not the smallest trace of the hereditary influence of the 

 foreign sperm which was used to evoke development. But of course, 

 throughout the animal kingdom, offspring are as likely to resemble 

 VOL. i c 



