BIGELOW: NUCLEAR CYCLE OF CONIONEMUS MURBACHII. 299 



The number of karyosomcs going to make up each segment is not con- 

 stant, but varies from two to four or five (Fig. (j). This is another reason 

 for believing that the karyosomes have no individual significance. It 

 has proved to be impossible to count the chromatin segments with abso- 

 lute accuracy; yet, from such counts as I have been able to make, there 

 is little doubt but that the number is twenty-four. This (Fig. 6) is the 

 latest stage in which the plasmatic portion of the nucleolus can be dem- 

 onstrated. Although there is little question of the plasmatic nature of 

 this body, it shows great individual variation in its reactions to iron 

 haematoxylin, sometimes staining intensety, at other times, even with 

 apparently the same treatment, hardly at all. 



It is difficult to trace the stages in the formation of the chromosomes, 

 because after the breaking down of the nuclear membrane the entire 

 reticulum becomes very much contracted. Certain phases of the process 

 are, however, clearly indicated. The membrane, distinct and stain- 

 ing deeply, persists until shortly after the formation of the chromatic 

 segments, and then disintegrates, the nucleolus also disappearing at the 

 same time. Cytoplasm now takes the place of the transparent fluid for- 

 merly filling the nucleus, and in it the reticulum lies in a contracted 

 condition (Fig. 8). This sudden change from the apparently inert nu- 

 clear sap to the much denser cytoplasm is undoubtedly of import- 

 ance in the later history of the nuclear elements. The segments now 

 appear denser and more deeply stained (Fig. 8) ; it is probable that this 

 condensation leads to the formation of the chromosomes, without any 

 further process of segmentation, so that the chromatin-segment stage 

 may thus be regarded as ontogenetically equivalent to the segmented 

 spireme of the higher Metazoa (see Fig. 8, which shows in a single nu- 

 cleus several stages in the process of condensation). Each chromosome 

 when fully formed exhibits a double or dumb-bell form, a condition found 

 also in the spermatogonia. Here however, each chromosome is formed 

 by the union of two chromatin masses, the chromomeres of Downing 

 (:05). In the eutoderm cells I have found no evidence that such 

 a union takes place, the method by which this peculiar form arises be- 

 ing here less apparent. The chromosomes still retain their connection 

 with one another through the linin strands. They now commence 

 to take up their positions in the equator of the cell, and this is 

 the most favorable opportunity for counting them. Even now, how- 

 ever, they are usually much crowded, so that an error of one or two is 

 always possible ; yet from an examination of several favorable cases I 

 believe that the number is in all probability twenty-four. Two such 



