^ SMALLWOOD : MATUKATION OF HAMINEA SOLITARIA. 273 



The chromatin masses are held together by the linin. Iii Figure 4 we 

 can see various places in the linin, where, if we imagine the network to 

 be the optical section of alveoli, we really have a series of vesicles. 

 That these are closed vesicles can be made probable by a comparison of 

 longitudinal and transverse sections, since in each case the limit is an 

 unbroken wall. 



At the same time that the linin disappears, these chromosome vesicles 

 increase in number. This would seem to indicate that there is a 

 genetic connection between the two. When the vesicles (vsl. cJifso.^ 

 are viewed in cross-section (Plates 3, 4, Figs. 14-17), one sees that they 

 are literally held in place by the linin, the linin being directly continu- 

 ous with the walls of the vesicle. This is evident only in the earlier 

 stages. 



It is at once evident that Haminea exhibits an unusual process in 

 regard to the formation of the chromosomes. The ultimate result is the 

 same as in other animals, but the intermediate steps are different from 

 those presented by the usual spireme. The chromosomes do not split 

 until the metaphase ; this is a strong argument for the view that no 

 regular spireme has been formed in the stages which precede the 

 earliest conditions observed. Wilson (:00, pp. 69-70) in describing 

 indirect division says : " The metaphase, which follows, forms the 

 initial phase of actual division. Each chromosome splits lengthwise 

 into two exactly similar halves, which afterwards diverge to opposite 

 poles of the spindle, and here each group of daughter-chromosomes 

 finally gives rise to a daughter-nucleus. In some cases the splitting of 

 the chromosomes cannot be seen until they have grouped themselves in 

 the equatorial plane of the spindle ; and it is only in this case that the 

 term ' metaphase ' can be applied to the mitotic figure as a whole. 

 In a large number of cases, however, the splitting may take place at 

 an earlier period in the spireme stage, or even, in a few cases, in the 

 reticulum of the mother-nucleus. Such variations do not, however, 

 affect the essential fact that the chromatic network is converted into a 

 thread ichich, whether continuous or discontinuous, splits throughout its 

 entire length into tico exactly equivalent halves." In discussing the 

 significance of the formation of the spireme, he adds (p. 245) the 

 following : " Roux argued that the facts of mitosis are only explicable 

 under the assumption that chromatin is not a uniform and homogeneous 

 substance, but differs qualitatively in different regions of the nucleus ; 

 that the collection of the chromatin into a thread, and its accurate divi- 

 sion into two halves, is meaningless unless the chromatin in different 



