58 bulletin: museum of comparative zoology. 



nucleus is shown by the recent experiments of Wilson (:01 b ) upon 

 etherized sea-urchin eggs. 



The later changes of the telophase — the disintegration of the chro- 

 mosomes and the formation of the nuclear membrane — are apparently 

 identical with those of the large type of spermatocyte. Indeed, most 

 of the changes in the succeeding division are quite strikingly similar to 

 those already described in the latter cells. This fact would at first 

 seem remarkable, when the dissimilarity of the earlier stages is consid- 

 ered, but, as I have said, these cells at all stages seem to be in a plastic 

 condition and very much under the control of surrounding influences. 

 The environmental conditions of the two types of spermatocytes during 

 the second spermatocyte stage are similar, the only discrepancies ap- 

 parently being a difference in the size of the cells themselves. 



The earlier changes of the prophase are so similar to those occurring 

 in the large type, that I think it hardly worth while to describe them 

 again in detail. The dyads arise in the same manner from the slightly 

 diffused chromosomes characteristic of the late telophase ; and the 

 ceutrosomes, after migrating apart along the cell membrane, take up 

 positions upon the cell membrane opposite to each other (Fig. 71). 

 The nuclear membrane is usually not re-formed, and the chromosomes 

 are drawn into the spindle where they are irregularly placed (Fig. 72). 

 They are often apposed to each other in such a manner that they 

 seem to be in the form of one or several convoluted bands. However, 

 there is certainly no real union of the chromatin during the second 

 spermatocyte stages, for by careful focussing the outlines of the in- 

 dividual elements can always be clearly seen. 



In shape and general characteristics the spindle resembles very closely 

 that of the large cells. The astral systems are well developed, but 

 while the rays are numerous, they are not as markedly so as in the 

 former type. It is noticeable, however, that the rays are arranged dif- 

 ferently from those in the first type, and this may indeed account for 

 the slight difference in the subsequent cleavage. As in the first sper- 

 matocyte, they cross in the equatorial region, but are attached to the 

 cell membrane in a very limited area. This brings it about that cleav- 

 age takes place without that extraordinary elongation of the cell char- 

 acteristic of the large type of the second spermatocyte. For, whereas in 

 the small spermatocyte the rays in the early metaphase are already 

 grouped in the plane of constriction, in the large type this grouping 

 must be accomplished by a further elongation of the cell in the manner 

 already described. 



