288 GENERAL EMBRYOLOGY 



then lies back of this, and determines the character of the pro- 

 gressive steps or reactions. This leads us directly to the second 

 chief view as to the fundamental character of the specific 

 organization of the ovum, that is, to the hypothesis of "nuclear 

 analysis" or nuclear determination, and to this we may now 

 give our attention. To state them again, the essentials of this 

 hypothesis are, that the real germinal localization of the ovum 

 is to be sought in the nucleus, that the organization of the cyto- 

 plasm is preceded and its character determined primarily, by 

 the organization of the nucleus, that this organization is 

 continuous from one generation to the next and is so to be 

 regarded as representing the physical basis of heredity. Polarity 

 and other cytoplasmic differentiations, certainly exist in the 

 ovum, even before fertilization or cleavage, but the only struc- 

 tural differentiation of the ovum which is invariably marked 

 out at all stages of the organism's existence, is the differentiation 

 between nucleus and cytoplasm. And while not alone develop- 

 ment, but all the normal life processes of the cell are the results 

 of interaction between nucleus and cytoplasm, both being 

 essential, yet the action of the nucleus is primary and seems to 

 determine the particularity of the cell actions. 



This general subject of nuclear determination is enormously 

 complex and has been the occasion for whole volumes; our 

 account of it must perforce be brief and therefore more or less 

 fragmentary and dogmatic. 



The search for the underlying causes of development is in 

 part a search for elements or conditions that are comparatively 

 fixed and that remain continuous from generation to generation 

 through the individual waves of species life. Specificity is 

 continuous; are there structural elements or conditions corre- 

 spondingly fixed and constant, not having to develop anew in 

 each individual ontogeny? Are there structures in the germ 

 cells which determine the direction of development and thus 

 represent (using this word in a very broad sense) the organs and 

 parts of the developing embryo? 



In the endeavor to answer these questions the nuclei of the 

 germ cells at once compel attention as containing organs whose 



