Q 



58 bulletin: museum of comparative zoology. 



stage is filled with dense karyoplasm, in which the chromatin persists in a 

 finely diffused condition, no nucleolus heing formed, and no archoplasmic 

 structures whatever being at this time visible in the cell. After the 

 nuclear membrane breaks down, the chromatic threads thicken and con- 

 dense to form the individual chromosomes. Unfortunately, the nuclear 

 contents at this stage are always so closely crowded that it has been im- 

 possible to count the chromatic elements; this has also been true of the 

 equatorial plate stage, end views of which were for some reason exceed- 

 ingly rare. In side views of this stage, however, it is evident that 

 the chromosomes are very large and comparatively few in number (Fig. 

 155) ; in side views of the anaphase (Fig. 156) it is even more appar- 

 ent that they are present in the daughter plates in a reduced number, 

 apparently much fewer than the somatic twenty-four. With their migra- 

 tion toward the poles typical daughter plates are formed (Fig. 156), and 

 in end views of these (Figs. 157 and 158), which are quite common, the 

 chromosomes can be counted more easily and more accurately than in 

 any other cells of Gonionemus. From a study of about ten such polar 

 views two very important facts result ; first, the number is without ex- 

 ception very much less than in the corresponding stage of somatic cells 

 or of the first cleavage spindle ; and secondly, the number of separate 

 chromatin structures is valuable, as are also their size and form. The 

 actual counts made ranged from twelve, or possibly thirteen, to fifteen. 

 In every instance the chromosomes are of different sizes, the larger ones 

 being clearly of dual nature, as is shown in the two examples repre- 

 sented in Figures 157 and 158. In those cases, where the larger number, 

 fourteen or fifteen, occur, two or more are invariably much smaller than 

 any of the others (Fig. 158). This evidence indicates very strongly, I 

 think, that the large chromatin bodies are double, each being formed by 

 the union of two small chromosomes, and that this union is not very 

 firm, the component portions of one or more of the dyads being readily 

 separated. Briefly stated, then, the result of this division is that each 

 daughter plate receives less than the somatic number of chromatic 

 elements ; a remarkable fact, the meaning of which can be discussed 

 more intelligently after the description of the later cleavages. 



3. The Third and Subsequent Cleavages. 



The material proved insufficient for a study of the third cleavage, but 

 when the fourth cleavage is reached, and, for that matter, all the subse- 

 quent mitoses, it is at once evident that the chromosomes are again 



