200 GENERAL EMBRYOLOGY 



cleavage once started in the right direction, leads then in a 

 perfectly natural and normal way to the later processes of de- 

 velopment. Loeb suggests farther that the spermatozoon may 

 contain certain substances, enzymes, which form within the 

 egg, materials capable of producing effects similar to these, 

 and that herein lies the natural stimulating effect of the 

 spermatozoon. 



Many facts regarding this hypothesis, both pro and con, have 

 been forthcoming in recent years, but it is still too early to say 

 how closely this approximates the truth. We might add, how- 

 ever, that Masing and others have not been able to detect any 

 marked synthesis of nuclein such a Loeb describes during 

 cleavage. And quite recently Conklin has determined that the 

 synthesis of nuclear substance is, in Crepidula, at least no 

 greater than the synthesis of cytoplasm. 



The eggs of other forms are more successfully treated by other 

 methods ; and each may have a particular treatment which is 

 most effective. But in very many of the instances of artificial 

 parthenogenesis the essential result of the treatment seems to be 

 a process of membrane formation accompanied by the with- 

 drawal of fluids from the egg. This has led to the suggestion 

 (Loeb) that the spermatozoon acts in this fashion, for it is 

 relatively very deficient in fluids, and upon entering the egg 

 reduces to some extent the relative fluidity of its cytoplasm, 

 thus acting as a stimulus. 



That the action of the spermatozoon is not specific, and that 

 fusion of the two germ nuclei is really not necessary to inaugu- 

 rate development, is clearly shown by the fact that almost any 

 spermatozoon, of whatever species, that can gain entrance to 

 an ovum, is capable of initiating development, and of effecting 

 the apparently normal cleavage of the ovum; to what extent 

 the internal processes of fertilization and cleavage are entirely 

 normal in such a case, we shall see later; suffice it to say here 

 that frequently a foreign sperm nucleus remains quiescent and 

 takes no part in the formation of the mitotic figure. 



It is true that the development of artificially fertilized ova 

 seldom proceeds farther than the cleavage stages. As a rule 



