THE SPERMATOGENESIS OF THE MYRIAPODS. 2O3 



die. The threads show a tendency to bend at a more or less 

 acute angle at this point, and this soon results in a transverse 

 division of each of the parts of the chromosome. Thus each of 

 the chrornatin segments has been divided into four parts and may 

 from now on be called a tetrad. Following the terminology 

 suggested by McClung, 'oo, I shall designate each of the parts 

 going to make up the tetrad or chromosome of the first sperma- 

 tocyte, a chromatid. By this system I believe much confusion will 

 be prevented. 



After the cross division has become established the next 

 change observable is shown in Fig. 1 3 b, c, g. The chromatids 

 revolve upon each other in such a manner that the ends at the 

 point of transverse cleavage are drawn out parallel to each other 

 and an irregular cross-shaped figure is thus formed (Fig. i$,d, c}. 



a 



* 



FIG. 13. X 1,400 dia. Various stages and modifications of tetrads, a, b, c, early 

 stages in the process of transverse division. </, typical tetrad of mid-prophase. e, J\ 

 g, h, modifications of the tetrad type. 



FIG. 14. X I 44 dia. Later stages in the history of the tetrad, a, typical cruci- 

 form tetrad of later prophase. b, "double V" form of chromosome at the same 

 stage, c, d, successively later stages of the cross figure, e, /, apparent modifications 

 of tetrad in later prophase. 2, //, typical chromosomes at beginning of metaphase. 

 g, tetrad undergoing longitudinal division. 



This cross-shaped figure is composed of four arms of about 

 equal length each of which is split longitudinally. Owing to the 

 very irregular shape of these arms, the cleavages are masked and 

 are often very hard to demonstrate. However, in later stages 

 when the arms are greatly shortened the bipartite structure is 

 readily seen (Fig. 14, a, b, c\ and is also strongly indicated even 

 in the earlier stages by the diamond-shaped opening at the center 



