292 GENERAL EMBRYOLOGY 



granules with a core of differentiated substance, probably linin; 

 in other cases the granules are arranged in a linear series wound 

 in a definite spiral; but in still other cases the granules seem 

 to have little if any regular disposition. These granules are 

 very close to the limit of vision, indeed often they are invisible, 

 and in most cases it is difficult to make confident assertions 

 regarding then- arrangement. Of course their invisibility on 

 account of minuteness does not prove that they are not present. 

 Indeed many, following Weismann, postulate an elaborate series 

 of representative particles within the nucleus : the chromosomes 

 or "idants," are divided into "ids" or chromatin granules; 

 the ids are then assumed to be formed of groups of "determi- 

 nants/ 7 and these in turn are thought to be composed of the 

 really elementary, self-propagating protoplasmic units, the 

 "biophores." Of this series, the idants and ids are visibly known; 

 the determinants and biophores are invisible hypothetical 

 bodies, postulated to aid in relating many of the complicated 

 facts of heredity to certain cytological facts. The assumption 

 of a final determining unit that is, and seemingly must remain, 

 invisible has proved fortunate as affording a convenient shelter 

 against criticism, for such an assumption partly removes the 

 question from scientific treatment. We shall lose little and gain 

 much by considering here merely those elements of the nucleus 

 that can be identified and whose behavior can be traced to 

 some extent, i.e., the chromosomes and chromatin granules. 



Our ideas in this field are, to a remarkable degree, the out- 

 growth of the pioneer work of Weismann. Although based 

 upon the properties of hypothetical units whose behavior was 

 outlined upon purely hypothetical grounds, his conceptions 

 of the relation between chromosome behavior and the facts of 

 development and heredity, formulated more than thirty years 

 ago, before the science of cytology was established, have a 

 distinctly modern aspect. The remarkable convergence of the 

 facts of heredity, of development, and of cytology, which have 

 become known subsequently to the formulation of Weismann's 

 hypotheses, constitutes splendid evidence of the keenness of 

 this great embryologist. 



