50 



SCIENCE 



[N. S. Vol. XLIII. No. 1093 



ness. For a normal fertilization reaction 

 certain internal conditions of the partners 

 and certain external conditions of the me- 

 dium must be realized. The study of the 

 external conditions throws light upon the 

 reaction, because the nature of the internal 

 conditions may be inferred from the neces- 

 sary, from the inhibiting, and from the fa- 

 voring conditions of the medium. 



External Factors. — The fertilization re- 

 action like all biological reactions requires 

 certain conditions of the environment, such 

 as definite range of temperature and chem- 

 ical composition of the medium. In the 

 first place, if these are exceeded in either 

 direction so far as to injure the cells the 

 fertilization reaction either does not take 

 place, or it is rendered abnormal. The 

 cause of the failure, or the abnormality, in 

 such cases lies in some change of the inter- 

 nal composition of one or the other of the 

 germ-cells. The classic experiments of this 

 kind are those of Oskar and Richard Hert- 

 wig published in 1887. These investigators 

 studied the effects of high temperature, 

 various injurious chemical reagents and of 

 mechanical shock on the germ-ceUs sepa- 

 rately before fertilization, and on the 

 process of fertilization itself at various 

 stages. Many exceedingly interesting ob- 

 servations were made, and problems were 

 raised that were not then ripe for solution. 

 Other experiments of a similar kind have 

 since been made, but their consideration 

 properly belongs to the problems of the 

 internal factors, for the phenomena ob- 

 served depend upon internal changes of 

 the germ-ceUs. 



In the second place there may be mod- 

 ifications of the medium which do not di- 

 rectly injure the germ-cells, but which in- 

 hibit or favor the fertilization reaction. 

 Examples of inhibiting phenomena are 

 found in Professor Loeb's studies of the 

 relations of ions to the fertilization re- 



action, or my own on the inhibiting action 

 of blood or tissue secretions of the same 

 species on fertilization. The most striking 

 example of conditions favoring fertilization 

 is the action of alkalis in enabling inter- 

 class hybridization, discovered by Jacques 

 Loeb. Such experiments furnish important 

 data for the analysis of the reaction, but it 

 is obvious that their interpretation must 

 depend upon internal conditions of the 

 fertilization reaction. 



In the third place the membranes of the 

 egg and of the spermatozoon must influence 

 the occurrence, rate and extent of the fer- 

 tilization reaction according to the degree 

 of their permeability to the substances con- 

 cerned ; the egg-membrane is of course more 

 especially concerned; its role in the occur- 

 rence of parthenogenesis has been studied 

 especially by R. S. Lillie ; and I have found 

 in the ease of the starfish egg that a resist- 

 ant egg-membrane may entirely block the 

 fertilization reaction, though the block 

 may be removed by agents that render the 

 membrane more permeable. 



The internal conditions of the fertiliza- 

 tion reaction may be grouped under two 

 heads: (1) Maturity of the germ-cells; (2) 

 specificity of the reaction. 



1. Maturity. — Concerning conditions of 

 maturity of the spermatozoon but little def- 

 inite is known, except that it will not ferti- 

 lize before its differentiation is complete. 

 Whether the cause of this lies entirely in 

 deficient motility, or partly also in incom- 

 plete chemical differentiation, we do not 

 know; though there are some reasons for 

 thinking that the latter factor may be in- 

 volved. In the case of the ovum our knowl- 

 edge is in a much more advanced stage. "We 

 know that the fertilizable condition, which 

 represents the final maturity of the ovum, 

 arises rather suddenly, usually lasts but a 

 short time, and is lost as an immediate eon- 

 sequence of the fertilization reaction. (a) 



