272 Cardiff: Synapsis and reduction 



From the time that the first explanation of reduction was 

 offered by Van Beneden, when he believed that half the chromo- 

 somes degenerated and were thrown off from the nucleus, until 

 the end of the past century, numerous conflicting views, which 

 need not be reviewed here in detail, were held. It suffices to say 

 that the stimulus to investigation as a result of Weismann's specu- 

 lations in regard to the chromatin in 1887 produced an immense 

 number of valuable results, some of the most important of which 

 may be mentioned. - 



Henking ('91) first figured a tetrad, or ring, which he inter- 

 preted as a chromosome undergoing one transverse and one lon- 

 gitudinal division, thus giving a true reduction division required 

 by Weismann's theory. Later tetrads receiving the same inter- 

 pretation were found in animals by Haecker ('9$a), Paulmier ('99), 

 Ruckert ('94), vom Rath ('92), and others. Calkins ('97) figured 

 tetrads in Pteris and Adiantum and interpreted them as a transverse 

 and a longitudinal division. In the same year Osterhout ('97) 

 figured the same in Eqitisetnm. Belajeff ('98) and Atkinson ('99) 

 figured a longitudinal and a transverse division in plant cells. 

 Though different observers found minor variations in the details 

 of tetrad formation, all agreed on the essentials ; namely, that 

 there was a longitudinal division of the chromatin thread followed 

 shortly by a transverse division, thereby forming a number of 

 segments equal to one half the number of somatic chromosomes. 

 These segments were looked upon as each composed of two 

 somatic chromosomes united end to end. In the ensuing divisions 

 these segments were divided once longitudinally and once trans- 

 versely, thus giving a qualitative reduction in accordance with 

 Weismann's hypothesis. 



On the other hand, the researches of Carnoy ('86), Boven 

 ('87), Hertwig ('90), and especially Brauer ('93) on Ascaris led 

 them to conclude that both divisions in tedrad formation were lon- 

 gitudinal ; that is, the tetrads arose by a double longitudinal di- 

 vision instead of by a longitudinal and a transverse division. The 

 -conditions in Ascaris, however, are complicated from the fact that 

 the number of chromosomes in the somatic cells is very much 

 larger than in the germ cells ; therefore, it may be questioned 

 whether the chromatin body dealt with in the germ cells is a true 



