lyo COMPARATIVE PHYSIOLOGY 



related by common ancestry, and with the attempt to trace 

 out the significant factors which have directed their past 

 history on the earth in the light of the evolutionary^ hypo- 

 thesis. As the treatment of the evolutionary problem forms the 

 subject matter of a separate volume in this series, we must here 

 confine ourselves solely to those questions which are amenable 

 to quantitative analysis, leaving out of account issues which 

 bear specifically on evolutionary biology. One may in passing 

 legitimately comment upon the importance still attached by 

 many physiologists to Darwinian concepts, a fact which is 

 surprising when it is remembered that the exact study of these 

 problems does not begin till the dawn of the present century ; 

 that it was not till more than ten years after the issue of the 

 *' Origin of Species " that the fertilisation of the egg by a single 

 sperm was clearly established ; and that the material available 

 for the study of inheritance by Darwin's contemporaries was 

 largely derived from popular tradition current among stock- 

 breeders. 



The natural starting-point for a study of the physiology 

 of reproduction is the fertilisation of the egg. The important 

 fact that the normal process of fertilisation involves the union 

 of only one gamete of either sex was first clearly established 

 by Hertwig and Fol (1875). The recognition of this fact 

 raises two problems. The entry of the sperm into the egg 

 normally implies (i) the initiation in the egg of active cell- 

 division culminating in the formation of a new individual ; 

 (2) the transference to the zygote of something in virtue of 

 which the nev/ individual so formed resembles the male as 

 much as it does the female parent. Kupelwieser (19 12) 

 found that with sufficiently long exposure of the egg and high 

 concentration of the sperm, it was possible to bring about the 

 development of a sea-urchin egg Vvdth the sperm of the common 

 mussel. The offspring reared resembled the former parent 

 only. Though the sperm was able to penetrate the egg, its 

 nucleus was eliminated during the subsequent cell- divisions, 

 and it therefore made no contribution to the hereditary con- 

 stitution of the fertilisation product. Hence, though it is not 

 the rule in nature that a sperm can supply the stimulus that 



