170 CONJUGATION, MATURATION, AND FERTILIZATION 



In simple division by mitosis each of the chromosomes divides 

 longitudinally, so that the daughter cells obtain the same number, and 

 an equal part of each chromosome. In 1883 Roux pointed out that 

 this wonderful mechanism in the cell, and the extreme care with which 

 each of the chromosomes is equally divided, must be connected in some 

 way with the phenomena of inheritance. Yan Beneden, in the same 

 year, first showed that the number of chromosomes in the uniting 

 nuclei is just one-half that of ordinary tissue cells of the body, the 

 number characteristic of the species being restored by the union of the 

 two halves, half from one parent, half from the otli^r. Weismann, in 

 his brilliant essays on heredity, suggested that each chromosome is 

 composed of a number of units, which he called biophores, each unit 

 representing some characteristic or group of characteristics to be 

 manifested in the prospective individual. These units are divided in 

 ordinary mitosis in such a way that each daughter cell would receive a 

 portion of each biophore, a result that could be reached only by longi- 

 tudinal division of the chromosome. To account for the differences in 

 characteristics of different offspring, he prophesied that in the forma- 

 tion of the germ cells the ordinary longitudinal division of the chromo- 

 somes would be replaced by a transverse division, and thus daughter 

 cells would result with different biophores. The apparent confirmation 

 of this prophecy a few years later was one of the great events in 

 the history of biology and a vast literature has accumulated since 

 1891 on this subject of "reduction." Today it is generally admitted 

 by cytologists that the reduced number of chromosomes is brought 

 about by association of the ordinary chromosomes in pairs (synapsis), 

 the union taking place by end to end or side to side association (telo- 

 synapsis and parasynapsis).^ In the preparation for fertilization such 

 double chromosomes are divided twice, so that four germinal elements 

 are produced from each primordial germinal cell. Furthermore, it 

 was suggested by Montgomery that the chromosomes representing 

 the same groups of characters are present in duplicate in the nucleus, 

 half coming from the male parent, half from the female, while synapsis 

 is the association of chromosomes representing the same groups of 

 characters but from different parents. This suggestion was rendered 

 more probable by the observations of Sutton, INIcClung, and others, 

 who showed that the chromosomes in insects have different forms and 

 that the different forms are present in pairs, and, further, that these 

 pairs unite in synapsis. 



There is reason to believe, therefore, that Weismann's original 

 hypothesis of the make-up of the germinal chromosomes is as close as 



1 These excellent terms were first used by Professor Wilson in lectures at Columbia Uni- 

 versity, and were u;sed by the present author and Miss Cull with the mistaken impression 

 that Professor Wilson had already published them. For this breach I offer my tardy but 

 sincere apologies. — G. N. C. 



