BULLETIN OF THE UNITED STATES FISH COMMISSION. 189 



nucleus. As soon as tlie segmentation lias been effected the granular 

 threads are withdrawn from the nodal points at the place where the seg- 

 mentation fiu^row severed them, and are finallj- 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 of the two new cells, and, as soon as the granular threads are withdrawn 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 

 through the same process, as here described, in the course of subse- 

 quent cleavages. This alternate elongation of nuclei into a spindle- 

 form, and contraction into a spherical 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 any one that this is essentially 

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

 his methods of work visible. 



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

 tion. Inasmuch as we have lowered the temperature of the air and 

 water, the media in which the ova of the shad underwent their devel- 

 opment, and find that it is retarded in consequence, we must naturally 

 conclude that the rate of segmentation, upon which the rate of develop- 

 ment directly depends, has been in some way interfered with or impeded 

 in its progress. Since we also saw that the rhythmical metamori)hoses 

 of the nuclei were directly concerned in the process of segmentation — 

 that in them the vis essentialis, essential force of segmentation, really 

 resides — it appears to me that we are also really bound to conclude 

 that the fall in the temperature has affected the activity of this vis essen- 

 tialis of the nuclei, which are retarded in their metamorphoses, in con- 

 sequence of which the rate of segmentation and development is retarded. 

 This fully and clearly accounts for the resulting prolongation of the 

 normal period of development when the temperature of the media in 

 which the ova undergo their evolution is lowered as much as is consist- 

 ent with their regular, healthful incubation. 



If retardation is possible, it ought also to be possible to accelerate de- 

 velopment. For centuries it has been the practice to accelerate and 

 maintain the growth of plants in hot-houses and forcing pits during in- 

 clement seasons of the year. This is proof enough, as far as the vegeta- 

 ble kingdom is concerned, that acceleration of the processes of growth, 

 which simply means that the acceleration of fissiparous cellular prolifer- 

 ation or segmentation is here possible. Its philosophj' is the same in 

 principle as that of retardation ; acceleration is the converse or recipro- 

 cal principle as opjjosed to the former. According to a table given by 

 Mr. E. E. Earll, in his paper on the development of the cod, in the 

 United States Fish Commissioner's report for 1878, page 721, we learn 

 that the minimuoi time of incubation for the ova of this fish is 13 days, 

 temperature of sea-water 10° F. ; the maximum time, according to 



