MATURATION MITOSIS (N AMPHIBIA. 26 I 



in the synapsis stage of the growth period before the maturation 

 divisions. Now what has to be determined is the changes that 

 occur in these chromosomes in the prophases of the first matura- 

 tion mitosis. In the early prophases (Figs. 2, 3, 5) these chro- 

 mosomes always show a definite arrangement ; in the figures 

 only a few of the chromosomes are shown in each case, namely 

 those which are seen in their entirety in the plane of the section. 

 There is a distinct polarity of cell body and of chromosomes, 

 and it is the same polarity which I found to obtain in the sper- 

 matocytes of Peripatns : the nucleus lies in the portion of the 

 cell where there is the least amount of protoplasm, and the 

 sphere (idiozome body) at that side of the nucleus directed 

 towards the greatest protoplasmic mass. For these two poles, as 

 a translation of Rabl's " Pol " and " Gegenpol," I used in the 

 case of Peripatns the terms " central pole " and " distal pole," 

 and these terms may equally well be applied to the amphibian 

 spermatocytes. The arrangement in the case of the chromo- 

 somes is well shown in the Figs 2, 3 and 5. Each chromo- 

 some has the form of a loop like a U or a V with the bend or 

 angle of the chromosome pointing towards the central pole, and 

 the free ends terminating at the distal pole of the nucleus; some- 

 times at these early stages the two free ends of a chromosome 

 may be applied together so that the whole chromosome has the 

 form of an elongated ring ; but generally in these early prophases 

 the U or V shape is the prevalent one. 



There are just twelve of these looped chromosomes, half the 

 number of those in the spermatogonia, as may be determined 

 by study of cells cut in a plane at right angles to the axis 

 connecting the central and distal poles. Thus in Fig. 4 (corre- 

 sponding to the stage of Fig. 3) can be counted twenty-four 

 cross-sections of chromatin threads, every two of which repre- 

 sent the two arms of one of the twelve looped chromosomes. 

 Now the point of great importance is that these early U and V- 

 shaped chromosomes have not arisen by any longitudinal split- 

 ting of a single chromosome, for in the very earliest prophases, 

 even earlier than that represented in Fig. 2, they have this shape ; 

 therefore the space circumscribed by the two arms of the chro- 

 mosome does not represent a longitudinal split, but a longitudi- 



