174 OUTLINES OF EVOLUTIONARY BIOLOGY 



In this way Weismann seeks to avoid the necessity of believing 

 in the transmission from parent to offspring of modifications 

 which result from the direct action of the environment upon the 

 body itself, without, however, altogether denying that external 

 influences, and especially nutrition, may act upon the germ plasm 

 through the body and thus cause modifications in the offspring. 



We have seen that Weismann's theory of the continuity of the 

 germ plasm involves the acceptance of the chromatin of the 

 nucleus as the actual material basis of hereditary transmission. 

 There can be no doubt that the phenomena of ordinary mitosis 

 in the case of tissue cells, and those of maturation and fertiliza- 

 tion in the case of the germ cells, strongly support this view, 

 while the simple fact that the spermatozoon, consisting almost 

 entirely of chromatin substance and with a minimum of cyto- 

 plasm, contributes equally with the ovum to the characters of the 

 offspring in normal cases, seems almost conclusive as to the pre- 

 dominating importance of the chromatin in this respect. Some 

 observers, however, still maintain that the cytoplasm plays a very 

 important part in heredity. 



It is probable that much light will be thrown upon this 

 question by the development of that extremely important branch 

 of biological science known as experimental embryology, which is 

 as yet in its infancy. We have already pointed out that in certain 

 cases eggs containing no nucleus may be fertilized by spermatozoa, 

 and may then develop up to a certain point, and that this process 

 is termed merogony. Some years ago Boveri claimed to have 

 fertilized enucleate fragments of the eggs of one genus of .sea- 

 urchins (Sphrerechinus) with the sperm of another genus 

 (Echinus), and obtained larvae with only paternal characters. 

 He concluded from this experiment that the nuclear substance is 

 alone responsible for the transmission of inherited characters. 

 Unfortunately it seems that his results are open to a different 

 interpretation, and they have been severely criticized. They 

 certainly cannot be regarded as by any means conclusive. 



More recently Godlewski has succeeded in fertilizing eggs of 

 the common sea-urchin (Echinus) with sperm of the feather star 

 (Antedon), belonging not only to a distinct genus but to a widely 



same advantages or disadvantages of position. It seems much more likely that 

 variations in this respect in different ids would tend to neutralize one another, the 

 kind of determinant which is unfavourably situated in one id being favourably 

 situated in another, so that each kind would, on an average, have the same chance 

 of nutrition. 



