336 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



iug tlie disk for some time in acid carmine, the nuclei were very dis- 

 tinctly brought out, so that all the phases of change which they un- 

 dergo could be observed in different individual cells composing the disk. 

 In some cells the nuclei were in the resting stage; in fact, such was the 

 case with the majority, and a comparatively small number were observed 

 to be undergoing cleavage with the nuclear matter aggregated at oppo- 

 site ends of the now elongating nuclear figure. Every r>hase of nuclear 

 metamorphosis could be studied deliberately after I had made perma- 

 nent balsam preparations, and from these I have been enabled to make 

 a number of camera sketches, which are almost as complex as any 

 figured by Flemming. These sketches are appended in an accompany- 

 ing plate. 



It is not known in what respect the nucleus differs chemically from 

 the rest of the protoplasm of the cell in which it is embedded, but it is 

 known that it is in some way very intimately concerned in the process 

 of cell-division, a phenomenon which always accompanies growth and 

 development. In the case of almost all, perhaps all orgaoisms, the cells 

 which compose them are constituted of a central more or less refringent 

 body, the nucleus involved in a covering of protoplasm usually more or 

 less different in optical properties from that composing the former. 

 Huxley observes (Anat. Invert. 19), "when nucleated cells divide, the 

 division of the nucleus, as a rule, precedes that of the whole cell." This 

 appears to be the fact in the case of the segmentation of the cells of the 

 germinal disk of the egg of the salmon, but just what sbare the envel- 

 oping protoplasm may take in the process we do not certainly know; 

 doubtless the enveloping protoplasm is just as necessary as an aid or 

 accessory in the process as the nucleus itself; they are probably in some 

 way complementary to each other. To support this view we have the 

 additional fact that we know nothing of absolutely naked nuclei, nor is 

 the evidence in favor of absolutely non-nucleated cells much more than 

 negative. The constancy with which complex cleavage-figures have re- 

 cently been observed in plants, Infusoria, Protoplasta, 3IoUusca, Echi- 

 nodermata, Arthropoda, and Vertehrata, both in the adult and embryo 

 condition, would argue that they invariably accompany and character- 

 ize cellular fission or segmentation, and, inasmuch as the rei)utedly non- 

 nuclcated Monera propagate by fission, pass through a resting stage 

 just like the cells of other organisms, it would not be surprising to find 

 that they too were provided with nuclei which would develop cleavage- 

 figures in the act of fission. Their complete parity of behavior in all 

 other respects leads us to look for important revelations respecting their 

 complete morphological corresi)ondence with other types of cells as soon 

 as they are studied by the help of improved methods. 



Having given the preceding sketch of the function and characteristics 

 of nuclei throughout the animal kingdom, we are prepared to deal some- 

 what more intelligently with the special case of the cleavage- figures 

 which we have observed in the ova of the salmon. The changes ob- 



