480 D. H. WENRICH 



When we examine a particular chromosome as it appears in 

 different cells we do not always see a consistent progress in de- 

 velopment in the successively later stages. Chromosome 4 in 

 cell E seems to be in about the same condition, so far as chroma- 

 tid movement is concerned, as it is in cell C; that is, either one 

 could, by the simple rotation of the free arms, assume the con- 

 dition seen in cell G. However, it is just possible that in cell 

 D a valid intermediate stage is represented and that this chromo- 

 some in the condition found in cell C would have to pass through 

 a stage such as that in cell D before reaching the condition seen 

 in F or G. 



e. The planes of the first division. In the absence of well- 

 marked polar granules it is impossible to determine which are the 

 proximal, or synaptic, ends of the chromatids, but if we may 

 assume for the time being that the ends at the left in cells B 

 to E are the synaptic ends, we may trace for chromosome 4 suc- 

 cessive steps as follows: In cell B the longitudinal split on the 

 right (the only one \dsible for two-thirds the length of the 

 tetrad) is probably the primary longitudinal split. The split 

 just forming at the left and continued to the point a is probably 

 the secondary split. In cell C, then, we may consider that the 

 formation of the secondary split has been completed and that the 

 chromatids have separated along this split almost to the distal 

 end. . Such an element might remain in this condition, con- 

 densing through a stage such as is shown in cell E and finally 

 opening out to form an element in the metaphase like the one in 

 cell G. In cell D we may have a stage in which the secondary 

 split has become complete but the separation along its plane has 

 not. If the progress of separation along this plane were to be 

 impeded by gi^adual condensation, this stage might be followed 

 by a cross formed by the proximal ends (left) of the two pairs of 

 chromatids becoming extended in opposite directions in the same 

 plane (as shown in chromosome 2, cell D), which cross might 

 reach the metaphase in some such form as shown for chromo- 

 some 5 and 6, cell F, or even chromosome 4, same cell. A simi- 

 lar series of stages occurs for chromosome 2 in cells B, C, and D. 

 Series of movements such as have been suggested would corre- 



