338 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



either side of its equator. In Fig. 5 they are very irregular ; iu Fig. 6 

 much more regular. 



Fig. 18 represents another phase of the progressive stage, in which 

 the refriugent nuclear rods at the opposite poles of the nucleus form a 

 loaf-like tigure. In this phase the rods seem to be about ready to part 

 at the equator so as to assume the arrangement observed in Figs. 19 

 and 20. These are also progressive and apiiroximating the conditions 

 shown in Figs. 13 and 15. Figs. 14, 16, and 17 are still further stages 

 in the process and indicate the stage when the cleavage or segmenta- 

 tion may be said to have been completed. In Figs. 9, 10, and 13 to 20, 

 inclusive, it may be observed that the refringent nuclear rods have been 

 aggregated at the poles of the nucleus, where they lie in a bundle some- 

 what like a bunch of fagots, or in the form of a crown or wreath. As 

 the polar crown-like figures are separated the refringent rods of which 

 they are composed are more and more compacted together into a closer 

 bundle, as indicated in Figs. 14, 10, and 17. 



In Fig. 21 the cleavage seems to have been completed, as the two 

 polar nuclear aggregations lie more nearly in the centers of the two new 

 cells. There is still a trace of the rod-like condition of the nuclear mat- 

 ter discernible, but the tendency is retrogressive, that is, to the phase 

 indicated in Fig. 2. This apparently completes the series of rhythmical 

 phenomena which we have observed in the cleavage of the nuclei and 

 cells of the germinal disk of the egg of the common salmon. In Fig. 21 

 the two new nuclei will undergo a retrogressive metamorphosis until 

 they are like Fig. 1 ; when this has been done, they will pass into the 

 so-called resting stage, only to repeat the series of changes of form and 

 rearrangement of their substance iu subsequent cleavages as described 

 above. 



In my paper on the retardation of development of the eggs of the 

 shad I have endeavored to explain why it was that segmentation ap- 

 peared to go on rhythmically ; data similar to these were there laid under 

 contribution to explain away the fact. I had, uj) to that time, not been 

 able to make any observations of my own on the behavior of the nu- 

 cleus during the acts of segmentation, and this notice is designed to 

 supplement and reinforce the arguments there put forth. As far as I 

 am aware, the foregoing account is the first which minutely describes 

 the behavior of nuclei iu the segmenting germinal disk of any fish, and 

 for that matter, the segmentation of auy of the cells of a teleostean. 



The stages represented in Figs. 3, 5, 0, 7, 8, 11, 12, and 18 appear to 

 represent, in successive order as named, the systole of the nuclear mat- 

 ter towards the equator of the nucleus, while Figs. 18, 20, 19, 10, 9, 13, 

 15, 14, IG, and 17 similarly and successively represent the <?mfo/e phases 

 of repulsion from the nuclear plate or equator. The rhythmical nature 

 of the phenomena can, I think, hardly be questioned, nor will auy one 

 doubt the propriety of the application of the terms systole and diastole 

 as suggested by Flemming. Lest it be objected that the nuclear figures 



