70 GENERAL EMBRYOLOGY 



In connection with the chromosomes there remain to be mentioned 

 two important hypotheses. The first is the hypothesis of the speci- 

 ficity of the chromosomes. Stated in its barest form the essence of 

 this idea is that each chromosome functions, in cell life, in its own 

 particular way, representing a center for reactions of a specific kind 

 only; that the chromosomes are cell "organs," functionally differentiated 

 and representing a division of labor roughly analogous to the functional 

 differentiations of the whole animal body. It is impossible to discuss 

 this hypothesis satisfactorily here and it is deferred to Chapter VII, 

 where it occupies a natural place in our account of the mechanism of 

 differentiation. 



The second hypothesis is that of the genetic continuity of the chromo- 

 somes. The essential of this idea is that the chromosomes which appear 

 during the preparation for a mitosis, are definitely related in a precise 

 way, to the chromosomes entering that nucleus at the close of the 

 preceding division. In its first form this hypothesis was called that 

 of the individuality of the chromosomes (Rabl, Boveri), and it was 

 held that the chromosomes actually, though not visibly, preserve 

 their structural identity during the period of interkinesis, that the 

 chromosomes of one mitosis are not related to those of the preceding 

 division, but are actually the same chromosomes. The fact that little 

 or no direct evidence of chromosomal identity during the interkinesis, 

 is to be had, has led to the remodeling of the idea of individuality, 

 into the hypothesis of genetic continuity. 



The nature of the evidence bearing upon this hypothesis, while not 

 scanty, is largely circumstantial, and hardly affords definite proof, either 

 affirmatively or negatively. We may suggest some of this evidence 

 without pretending to give a detailed account of the facts. 



At the very beginning we must recognize that the chromosomes are 

 actually visible, as differentiated structures, only during that com- 

 paratively brief period of cell life occupied by the process of mitosis. 

 Considering first some of the facts opposed to this hypothesis, we should 

 say that those who deny the fact of continuity, maintain that during 

 the vegetative period of cell life the dissolution and disappearance of the 

 chromosomes is not only apparent but real. There is truly little visible 

 and direct evidence of chromosomes during interkinesis. And further, 

 much new chromatin is formed during this period which cannot be dis- 

 tinguished from the chromosomal chromatin; in the early stages of 

 mitosis much chromatin is again thrown out of the nucleus and takes 

 no part in the formation of chromosomes. It is impossible to say 

 whether the chromatin forming the chromosomes is or is not then, the 

 same as that previously derived from them, or that resulting from growth. 

 Many instances are known where the chromatin of the vegetative 

 nucleus is nearly all contained within a single homogeneous chromatin 



