Cardiff : Synapsis and reduction 28o 



Finally there is a decided shortening (figure 20) of the threads and 

 the paired portions show more of a tendency to separate. It is 

 about this stage that we have the first evidence of a transverse seg- 

 mentation of the threads into chromosomes. As the chromosomes 

 pass through the prophase of the first, or heterotypic, division 

 they are always bivalent in character and at first the parts of each 

 bivalent are often twisted several times upon each other and they 

 are either in contact with the nuclear membrane or close to it. 

 During the prophase the parts of the bivalents untwist so as to 

 give rings, 8's, Y's, V's, X's, and the various other forms so 

 characteristic of this stage (figures 21-32). It is easy to see how 

 these forms have been so often interpreted as tetrads formed by a 

 longitudinal and a transverse division. There is here, however, 

 not the slightest evidence of a transverse divison of the heterotypic 

 chromosomes ; on the other hand, the evidence seems strong that 

 there is simply a separation of univalent parts of a bivalent chro- 

 mosome which attained its bivalent character in synapsis. From 

 the time they are formed the chromosomes gradually become 

 shorter and thicker until metaphase is reached when they are 

 almost spherical in shape. After synapsis is reached the nucleus 

 ceases to increase in size. The cytoplasm immediately surround- 

 ing the nucleus becomes much more dense and shows a decided 

 fibrillar character (figure 1 5), probably the beginning of the spindle- 

 fibers. As metaphase of the first division of the mother-cell is 

 reached the chromosomes are closely crowded together and the 

 spindle is very small though very clearly defined (figure 35). 

 Repeated counts of the chromosomes both in this and the entire 

 prophases (figures 33 and 34) indicate that the number is probably 

 eleven. Individual differences in the size and shape of the chro- 

 mosomes were evident in nearly all nuclei. Some were three 

 times the size of others (figures 33 and 34). Certain chromosomes 

 by their shape and size could almost always be identified in the 

 different nuclei. It will require further work to determine what 

 may be the significance of these size differences, but that they 

 are constant there seems little doubt. 



From metaphase of the heterotypic division to the final spore 

 formation the stages are passed through very rapidly. There seems 

 no doubt that the second division is homotypic in character. 



