FERTILIZATION 207 



the lower the organism in the evolutionary series, the farther 

 its development may proceed. And while some artificially 

 fertilized Echinoderm eggs have been carried past the larval 

 stage (Delage), the Chordate ovum (Teleost, Cyclostome, 

 Urodele) will cleave only a few times. This indicates clearly 

 that the parallelism between natural and artificial fertilization 

 is not complete, although it is not unlikely that ultimately a 

 form of treatment may be found which will produce just the 

 same result as normal fertilization, save in so far as this is con- 

 cerned in the inheritance of individual characteristics. 



In all cases of artificial parthenogenesis the cleavage figures 



o 



are essentially normal except that the reduced or ^ number 



of chromosomes is present (exceptions to this have been 

 reported by Tennent and Hogue, and others) ; the poles of the 

 spindle are occupied by typical centrosomes, formed anew by 

 the substance of the egg after the disappearance of the oocyte 

 centrosomes of the maturation spindle. This is also true re- 

 garding the centrosomes of normally parthenogenetic eggs. 

 And there are several instances known where the specific effect 

 of certain reagents or external conditions is the formation, out 

 of the cytoplasm of the ovum, of numerous centrosomes, 

 apparently of normal structure and each with a small aster 

 (Yatsu). 



To summarize the evidence from parthenogenesis, both nor- 

 mal and artificial, we may say that, among the Metazoa, the 

 ovum contains within itself a mechanism sufficiently complete 

 to function for a time at least, although the spermatozoon, 

 when it enters, does add to this mechanism and supplies some 

 parts not present in the egg; these parts either are not abso- 

 lutely necessary or they may, under certain conditions, be 

 supplied from the structure of the ovum in the absence of the 

 sperm. And further, while the egg may be stimulated to 

 develop by means other than the entrance of the sperm, this 

 is normally the form of stimulus which inaugurates the series 

 of reactions we call development. Taking this view of fertiliza- 

 tion the formation of the spermatozoon is a means of insur- 



