THE CELL-DOCTRINE 127 



been searchingly examined. The question has been asked 

 whether any one portion of the germ-cell is more important 

 than another. The answer is surprisingly definite. The 

 material composing the nucleus rather than the cytoplasm 

 seems the primary agency in development (Fig. 11). And 

 within the nucleoplasm a stainable substance, the chromatin, 

 is the more important. In the intervals between cell divi- 

 sions the chromatin appears to exist in the form of stainable 

 granules. But with the onset of cell division it takes the 

 form of definite bodies, the chromosomes (Fig. 13 B, C, D). 

 Although the chromosomes have been traced through the 

 non-dividing period in only a few instances, there are 

 theoretical reasons for believing that their individual exist- 

 ence is always preserved during these intervals when they 

 cannot be readily recognized as chromosomes (Fig. 13 A 

 and F). Chromatin, at least, persists and from it chromo- 

 somes are formed at every period of division. What is 

 termed the Chromosome Theory of heredity has been devel- 

 oped, because the behavior of the chromosomes is correlated 

 with end results in inheritance. 



The more general evidence, which points to the chromo- 

 somes as the bearers of the heritage, can be appreciated 

 upon brief explanation. When fully matured, the male and 

 female germ-cells are very dissimilar in appearance. The 

 ovum is, save for the presence of non-living food material 

 called yolk, a very typical cell (Fig. 12 A). The spermato- 

 zoon on the other hand is a most atypical cell (Fig. 14 A, 

 B, and D). It consists of a mass of condensed nuclear 

 material, almost wholly chromatin, and a relatively small 

 amount of cytoplasm. At the time of fertilization, the so- 

 called head and the ?niddle-piece of the spermatozoon enter 

 the egg, while the tail or flagellum usually remains on the 

 outside playing no further part in development (Fig. 12 

 B and C) . While the middle-piece almost invariably enters 

 the ovum along with the head or nucleus, this seems un- 

 necessary even where it normally occurs. The essential 



