188 Harry Lewis Wieman. 



entangling of the chromatin thread which has remained practi- 

 cally unchanged from the telophases of the last pre-maturation 

 division, while the nuclear cavity has been enlarged. Wilson ('09a) 

 speaks of the nuclear reticulum in Pyrrochoris as undergoing 

 contraction, yet the actual contraction of the chromatic material 

 is rather insignificant (as shown by his figures. Photos. 35-42 

 inclusive) compared with the expansion of the nuclear membrane. 

 Likewise in the case at hand the chromatin has changed but 

 slightly in bulk since the telophases of the pre-maturation divisions, 

 and the apparent contraction seems to be due principally to an 

 enlargement or vacuolization of the nuclear cavity. 



Fig. 51 shows the characteristic appearance of the spireme of 

 the prophase of the first division, when the thickened thread 

 is composed of the reduced number of segments, each of which 

 is double. The nucleolus of the resting stage forms one end of 

 the series (Fig. 50), and its relation to the spireme is such as to 

 suggest that each segment of the latter is homologous with it. 

 That is to say, if the nucleolus represents two fused or closely 

 united chromosomes, then each segment of the spireme can be 

 regarded as a double chromosome. In other words, synapsis 

 has taken place by side to side union of the chromosomes. On 

 the other hand, if we regard the nucleolus as a single chromosome, 

 then end to end union of the chromosomes has occurred; and in 

 this case the groove separating the two halves of the spireme 

 represents the line of a longitudinal division ; whereas in the other 

 it represents the line along which the chromosomes had come 

 together in synapsis. 



An examination of Figs. 46, 47, 40 and especially 42, shows that 

 during synizesis or at least very shortly after, the split spireme 

 bears a similar relation to the nucleolus. To each lobe of the latter 

 is attached one of a double series of segments, the number of 

 which cannot be counted, but which is more than the unreduced 

 number. In fact, there is approximately twice this number, 

 which means that the chromosomes had united to form a spireme 

 the demarcation of which into segments foretells the two matu- 

 ration divisions. 



Throughout the period preceding the first maturation division. 



