356 bulletin: museum of comparative zoology. 



light which it may throw on the role and subsequent fate of the mater- 

 nal and paternal chromatin in the cleavage nucleus, and because of its 

 importance in a comparison of the details of indirect division in larval 

 cells and in adult tissues. The eggs of Gonionemus in the stages which 

 I have at command have unfortunately proved to be very unsatisfactory 

 for the solution of the first of these questions ; but in the case of the 

 second they have disclosed certain features with regard to the numerical 

 condition of the chromosomes in the first, second, third, and fourth 

 cleavages which seem to me so remarkable as to merit description, al- 

 though some of the stages have not been worked out in as great detail 

 as I could wish. On whole eggs as transparent as those of Gonionemus 

 it is possible to trace fairly well the changes of the achromatic figure 

 and of the chromatin in active mitosis ; but the staining has proved 

 unsatisfactory for a resolution of the finer details existing in resting 

 nuclei, and in the prophase prior to the dissolution of the nuclear mem- 

 brane. The sections were not more helpful in these stages. 



1. The First Cleavage. 



The first cleavage was studied chiefly on eggs fixed in 40 per cent 

 formaldehyde and stained in borax carmine, only a very few corrosive- 

 acetic preparations being available. The first cleavage nucleus is repre- 

 sented in Figure 150. It does not pass into a resting stage, nor is a 

 nucleolus formed, but very soon after conjugation the nuclear membrane 

 breaks down and the nuclear structures then lie free in the cytoplasm 

 (Fig. 151). Previous to the dissolution of the membrane the chromatin 

 was assembled in a loose and irregular thread work (Fig. 150), but imme- 

 diately after that event a process of condensation takes place, by which 

 the threads thicken, become more clearly segmented, and considerably 

 less numerous. This is a phase similar to that already described in both 

 somatic cells and oogonia (pages 298 and 338) at a corresponding stage ; 

 it leads directly, through still further condensation, as in both those cell 

 generations, to the formation of the definitive chromosomes (Fig. 152). 

 These are rod-shaped, often somewhat dumb-bell-like, and usually lie 

 more or less separated from one another, so that in two instances I have 

 been able to count them with a fair degree of accuracy, the result in both 

 cases being twenty-four. They now arrange themselves in an equatorial 

 plate (Fig. 153). No centrosome can be distinguished, but there is to 

 be seen at the centre of the radiations a deeply-staining homogeneous area 

 (Fig. 153), which is perhaps comparable to a ceutrosphere. 



