20 PROBLEMS OF FERTILIZATION 



This conception exerted a dominating influence on 

 the series of fertilization studies which followed; the 

 questions as to the origin of the sperm aster with its 

 contained centrosome in the egg, and as to the genetic 

 continuity of the sperm centrosome with the centro- 

 somes of the cleavage amphiaster, were energetically 

 investigated by a series of students for the next fifteen 

 years or more, and similar studies have continued with 

 less energy down to the present time. Collectively 

 these publications constitute a fairly adequate record 

 of the morphology of the fertilization process in ani- 

 mals, a large part of which was furnished by American 

 students. 



The morphological analysis of fertilization seems 

 now to be fairly complete; there may still be disturb- 

 ances, such as recent attempts to trace mitochondria 

 back to the sperm, which seems destined to share the 

 adverse fate of the similar attempt to trace the centro- 

 somes to the sperm; but there is not likely to be any 

 great modification of the existing data, which seem to 

 me to demonstrate, effectively if not absolutely, that 

 the sperm head contains all the substances necessary for 

 fertilization. We have thus attained a more or less 

 definitive solution of the morphological relations of egg 

 and spermatozoon in the fertilization process. 



The cytologist working with chromosomes and the 

 geneticist with Mendelian factors have traced maternal 

 and paternal elements through the life-history in a 

 very satisfactory manner, so that we are beginning to 

 see how certain strands of the web of life cross the gap 

 of successive generations. It remains for the biology 

 of the future to elucidate the chemical foundations of 



