142 



M. \V BLACK. MAN 



phase and later during the metaphase differ to some extent. A- 

 this difference in shape is one characteristic by which we must 

 hope to identify the various chromosomes, occasion may be taken 

 here to describe briefly the processes by which these various 

 forms arise. 



What I shall call type A is represented in the text-figure I. 

 The origin and evolution of this type of tetrad is described in 

 sufficient detail in previous papers (Blackmail, :O3, 105, op. cit.); 

 so it will not be necessary to repeat the description in detail. 



a 



I 





/ 



h i 



FIG. I. Semi-diagrammatic representation shewing the formation and history 

 of the cross-shaped type of tetrads; a, bivalent chromatin segment as it appears 

 in the very early prophase; b, planes of longitudinal and transverse cleavage 

 established; c, d, later stage in evolution of prophase tetrad; e, f, tetrads as seen 

 in early metaphase; g, tetrad in act of division, showing the manner in which the 

 component parts glide over each other; h, early anaphase showing distortion of 

 halves of tetrad due to their close adhesion; ;, daughter chromosome in metaphase 

 of second spermatocyte. 



FIG. II. Corresponding stages in evolution of the clouble-V type of tetrad. 



I wish, however, to emphasize two points. First the points at 

 the ends of the shorter arms of the cross-like figure (Fig. I, b, c,d) 

 represents the point at which union occurred during synapsis. 

 The attachment of the mantle fibers in the metaphase is not at 

 this point as it is said to be in Syrbula by Robertson (:o8, op. cit.}, 

 but is at the ends of the longer arms of the cross as shown in 

 Fig. I, c,f, g. 



Robertson believes that the attachment of the mantle fibers 



