54 GENERAL EMBRYOLOGY 



the assumption that the chromosomes constitute the essential 

 physiological elements of the nucleus and therefore of the cell. 

 There are many subsidiary facts indicating the great importance 

 of these bodies. Consideration of the relation of the chromo- 

 somes to the special problems of heredity and sex will be deferred 

 to a later chapter (Chapter VII) for fuller consideration, but 

 we should mention here a few of the important facts and hypo- 

 theses regarding these bodies. 



In the nuclei of many of the Protozoa definite chromosomes 

 are already present, but in some unicellular organsims there are 

 conditions suggesting a possible mode of evolution of these 

 structures. We have mentioned the collection of the distrib- 

 uted chromatin granules into small groups through the cell; 

 these groups have been regarded as the rudiments of chromo- 

 somes. After a definite nucleus is established the chromatin 

 granules remain as definite bodies, and each divides in cell 

 fission. Even when the chromatin granules merely become 

 rearranged about a division center, as in Chilomonas and 

 Trachelomonas, although definite chromosomes may not be 

 formed, the division of the granules occurs as the essential step 

 in fission, just as later when the granules collect and fuse into 

 chromosomes. In Paramcecium definite chromosomes are 

 formed only in certain divisions, namely, those immediately 

 preceding conjugation, that is during garnet ogenesis (Calkins 

 and Cull, Fig. 82), and it is but a short step from this condition 

 to the regular formation of chromosomes in all mitoses. 



In the reconstruction of the daughter nucleus the chromo- 

 somes become vacuolated and finally break up into scattered 

 granules whose distribution through the nucleus is so irregular 

 that in nearly all cases no trace of chromosomal structure is 

 apparent. The nucleus then grows rapidly, the chromatin 

 content often increasing to many times that in the original 

 chromosome group, until it soon reaches a quite definite size, 

 varying widely in different cells, but fairly constant for cells of 

 a single kind in a given species. The nucleus and chromatin 

 then remain without much further change, quantitative at 

 any rate, until toward the close of the vegetative or normally 



