488 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [34] 



cleavage figures developed during the segmentation of the germinal 

 disk of tlie egg of the salmon," was the result of a study of a compara- 

 tively early stage. I have since assured myself by an investigation of 

 the more advanced blastoderms of the ova of the trout and white fish 

 that complex nuclear metamorphoses continue to manifest themselves 

 much later and are a constant accompaniment of cell division or mul- 

 tiplication in the blastoderm. 1 have also elsewhere pointexi out the 

 effect of fluctuations of temperature in accelerating or retar<liug the 

 division of nuclei, and consequently its influence upon the rate at which 

 development proceeds. Although these studies are anatomico physio- 

 logical in character, their bearing upon the labors of the Fish Commis- 

 sion are important in that they afford us a rational interpretation of a 

 very obscure process, viz, that of growth and development and its de- 

 pendence upon temperature in cold-blooded vertebrates. If it is agreed 

 that the force which determines development is affected by changes of 

 temperature, it must follow that growth force is in some way dependent 

 upon heat, one of the forms of molecular motion. The facts show 

 that this is true, and that growth,— cell division, appears as if^t might be 

 regarded as a form of physiological work exhibited by protoplasm un- 

 der the direction of determinate laws of nuclear change. It appears 

 that irregularity or asymmetry of development of the cells of the germ- 

 inal disk is common in meroblastic ova with a large yelk, as is note- 

 worthy in the eggs of birds, reptiles, Elasmobranch and Teleostean fishes, 

 especiallj'^ during the early stages. This appears to be dependent upon 

 the behavior of the nuclei after the disk has been segmented into sev- 

 eral cells. Their metamorphoses from some cause do not exactly keep 

 pace with each other ; some divide sooner than others, so that it results 

 that some pairs of incipient cells have already divided before division 

 has begun in others, giving rise to a disk composed of irregular and 

 unequal sized cells at the time when its cleavage into a single layer is 

 comi)leted. 



As already stated, the metamorphoses of the nuclei, which have in 

 reality descended from the first segmentation nucleus, are rhythmical. 

 At first round and containing, besides a nucleolus, numerous granules 

 and even granular reticuli, at the time segmentation is about to begin 

 its contents rearrange themselves ; the nucleus, in large cells of early 

 stages often showing as a clearer rounded body imbedded in the center 

 of the cellular protoplasm, slowly acquires a more elongate form ; at the 

 same time its granular contents tend to arrange themselves in bands 

 nearly at right angles to the plane of cleavage. These lines then seem 

 to undergo a further metamorphosis, in that their substance becomes 

 slowly more homogeneous and is finally aggregated into very refringent 

 rod like bodies arranged in the form of a wreath or crown at either pole 

 of the nucleus. These wreaths or crowns of refringent rods are then 

 rei)elled more and more from the plane of cleavage, and at the same 

 time tend to become more densely packed together in a parallel manner. 



