296 GENERAL EMBRYOLOGY 



of unit characters is far in excess of the number of chromosomes. 



That the chromosome of the Metazoan is really made up of a 

 group of unit determiners, is also indicated by the behavior of 

 the Protozoan nucleus in maturation. In most of the simpler 

 Protozoa where the maturation phenomena appear, there is no 

 indication of definite elements like chromosomes in the nucleus. 

 But in many of the Ciliates, in which vegetative chromatin and 

 reproductive chromatin become sharply separated, the latter, 

 or idiochromatin, is seen to be formed into definite bodies. Thus 

 in Paramcecium, as observed by Calkins and Cull (Fig. 82), the 

 micronucleus (idiochromatin) becomes resolved into a large 

 number more than 200 chromatin granules (idiochromidia) 

 whose definite behavior can be traced. Their behavior is com- 

 plex, but the result is that each idiochromidium is divided 

 longitudinally and transversely, and the resulting daughter- 

 bodies may, therefore, be dissimilar. After fertilization the 

 division of the zygotic nucleus brings about the division of each 

 chromidium and the distribution of the halves to the two 

 daughter cells. 



The very large number of separate elements in these "gam- 

 etic" nuclei may indicate that each corresponds with a single 

 character, or with a smaller group of characters than in the 

 Metazoan, and that therefore the chromosome of the Metazoan 

 must be an enormously complex affair. All of this lends weight 

 to the idea that "chromosomes, the characteristic structures of 

 the nucleus in mitosis, have had an evolution no less surely 

 than has the nervous system, digestive system, or supporting 

 system of the higher animals, and that the chromosomes of the 

 protozoa have the same relation to the chromosomes of the 

 metazoa that the organization of the protozoan body has to 

 that of the metazoan, i.e., a unit structure." (Calkins, " Proto- 

 zoology," page 171). Admitting the representative particle 

 composition of the chromosomes, it must of course follow that 

 their evolution in the Metazoa, parallels the evolution of adult 

 form and structure. 



If this is true, then the chromosomes of the Metazoan germ 

 cells must each represent a congeries of determiners, the form 



