[15] EE.TARDING DEVELOPMENT OF SHAD EGGS. 809 



unclear transformations consequently occur in the cellular elements of 

 tish embryos. These observations are further supported hj the fact 

 that both Brooks and myself have observed undoubted evidence of the 

 rythmical 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 rythmical 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 Lave nothing to do with these latter kind 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 

 I^rocess 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 consuumiated 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 i:)ass, 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 

 nucleus. As soon as the segmentation has been effected the granular 

 threads are withdrawn from the nodal points at the ])lace where the seg- 

 mentation furrow severed them, and are finally retracted into what were 

 formerly the two poles of the spindle. These poles are now the nuclei 

 of the two new cells, and as soon as the granular threads are withdrawn 

 towards these new polar nuclear centers the latter become globular and 

 pass into the resting stage. Afterwards they both elongate and go 

 ttirough the same process as here described in the course of subse- 

 quent cleavage. This alternate elongation of luiclei into a spindle- 

 form and contraction into a si)]ierical form in the process of cleavage 

 has been called by Flemming the diastole and systole of the nucleus. 

 They accompany the rhythmical phenomena of segmentation and give 

 us a rational and philosophical interpretation of the phenomena of seg- 

 mentation. It must. I think, be plain to anyone that this is essentially 

 a dynamic process, in which the Artisan of organization almost makes 

 Ilis methods of work visible. 



It also affords a scientific explanation of the phenomena of retarda- 



