BLACKMAN : THE SPERMATOGENESIS OF SCOLOPENDRA. 29 



granular threads, often considerably curved or distorted in various ways. 

 They rapidly become straighter and soon show signs of longitudinal 

 cleavage. In Figure 24 most of the chromatin segments are in this 

 condition, i. e. they consist of more or less curved granular rods each 

 of which has been longitudinally split into two parts. Since it is to 

 the prophase that we should look for indications as to whether the 

 first spermatocyte division is one of equation or reduction, it is of impor- 

 tance to note here that the first indications in the prophase point to a 

 longitudinal division. As the longitudinal division of the chromatin 

 segments is the first which occurs in the prophase, I think it very 

 probable that it is the one completed by the first maturation mitosis. 



"What is apparently the next change in the appearance of the. split 

 segments is shown in Figures 26, 27. This first becomes evident as a 

 " weakening " of the two parts of the segment at about their middle. 

 The threads show a tendency to bend at a more or less sharp angle at 

 this point (Fig. 26, A), and this soon results in a transverse division 

 of eacli of the parts of the chromosome. Thus, each of the chromatin 

 segments has been divided into four parts and may from now on be 

 called a tetrad. Following the terminology suggested by McClung, I 

 shall designate each of the parts going to make up the tetrad or chromo- 

 some of the first spermatocyte, a chromatid. By this I believe confusion 

 will be prevented. 



After transverse division has become established, the next change 

 observable is shown in Figure A, a, b, c. 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. A, d, e). This cross-shaped figure is 

 composed of four arms of about equal length, each of which is split longi- 

 tudinally. Owing to the very irregular shape of these arms, the cleav- 

 ages 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. B, a). It is also strongly indicated, even in the 

 earlier stage, by the diamond-shaped opening at the centre of the tetrads. 

 "When viewed en face this opening is always square or diamond-shaped, 

 with the angles directed toward the arms, indicating that the opening 

 is continuous into the arm. 



At the stage represented in Figure A, e, /, the tetrads are often so 

 distorted that the typical form is lost, but upon studying them more 

 carefully it is seen that they are always referable to the same type. 

 Taking d as the type, the more common variations are shown in b, c, e, 



