188 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



Strasburger on plants. Indeed, so striliingly is this true, that Stras- 

 burger has been tempted to utter the dictum, omnis nucleus e nucleo, 

 which in English means that all nuclei originate from pre-existing nuclei, 

 just as formerly Schwann expressed himself to the same effect in rela- 

 tion to the genesis of cells. Such intracellular granular networks ex- 

 tending outwards from the nucleus through the protoplasm enveloping 

 it may be seen well developed in the coarse vesicular connective tissue 

 cells of the American oyster, of which I have mounted preparations. 

 Vastly more complex intranuclear reticuli are found in the nucleus of 

 the unripe eggs of the common slipper-limpet, Crepidula glauca. I have 

 seen the granular threads in these undergoing the most wonderful active 

 changes of form. Spindle-shaped nuclei, the opposite poles of which 

 were joined by granular threads, have been observed in the eggs of 

 Elasmobranch fishes by Balfour. These were in the act of division, or 

 in the diastole condition spoken of by Flemming. CEllacher has seen 

 granular threads radiating from the nuclei embedded in the cells of the 

 germinal disk of the trout in its early stages of development. These 

 nuclear transformations consequently occur in the cellul.ir elements of 

 fish embryos. These observations are further supported by the fact 

 that both Brooks and myself have observed undoubted evidence of the 

 rhythmical nature of segmentation in fish ova, which ought to be the 

 fact, since it has been shown that the metamorphoses of the nuclei are 

 likewise rhythmical in character. 



The metamorphoses, or changes in the form and structure of the 

 nucleus, are, in large part, connected with the genesis of new cells, in 

 the successive acts of cleavage or segmentation ; their metamorphoses, 

 doubtless, also play an important part in the functions of rejuvenescence 

 and depuration of cells, or in the general functions, repair and waste, as 

 well as in the excretory and secretory functions of organs. But in re- 

 tardation we have nothing to do with these latter kinds of nuclear 

 metamorphosis; we are only concerned with the alternate elongation 

 and contraction of the nucleus attendant upon the process of segmen- 

 tation or the fissiparous genesis of new cells, in which the pre-existing 

 nucleus of a cell, about to divide, elongates, becomes severed into two 

 parts, which become, respectively, the nuclei of two new cells. In the 

 process of cleavage it has been shown that, during the act of cleavage, 

 the nucleus of the cleaving cell elongates, becomes spindle-shaped ; that 

 the opposite poles of the spindle become, respectively, the nuclei of the 

 two new cells resulting from the completed process of segmentation. 

 During the active stage the two poles of the spindle are joined by a 

 barrel or spindle-shaped series of granular threads. When the seg- 

 mentation is about to be consummated, these threads, half way between 

 the poles, are found to have developed nodes or swellings ; these mark 

 the point through which the segmentation furrow will pass, so as to sep- 

 arate the old cell into two new ones. The segmentation furrow, accord- 

 ingly, passes at right angles across the long axis of the spindle-shaped 



