[107] EMBRYOGRAPIIY OF OSSEOUS FISHES. 561 



and is therefore more <lei)ressed. This elevation and depression of the 

 upper surface of blastouieres in the course of segmentation is often a 

 very marked feature, and gives rise to the most singular superficial irreg- 

 ularity of the whole disk up to the time of the completion of the moiula 

 stage of development. These changes are doubtless due to interual 

 movements of the substance of the blastomeres, dependent upon an in- 

 terual radial contractile structure. 



The radial and reticulated structure of protoplasm is to Rauber of 

 the most i^rofound significance in relation to the phenomena of growth 

 and development. His conclusions are here, in part, reproduced : 



"1. Radial and trabecular structures of protoplasm are not essen- 

 tially distinct but are manifestations of the same principle, in that the 

 latter is developed as a result of vacuolization, the former in the di- 

 rection of readiest division {sjjaltharl-eit). 



"2. The radial and trabecular structure of animal as well as vege- 

 table protoi)iasm is a factor with which every investigation into the 

 history of the growth of an animal or a vegetable body has to do ; such 

 structure is vitall}' related to the beginning of development. 



"3. The radial and trabecular arrangement of protoplasm grows 

 both by the addition of new material from without, at its peiipheral 

 ends, as well as by the incorporation of such material within the pre- 

 existing substance, or both by apposition and intussusception. The 

 l)rotoplasmic streaming necessary for this purpose is facilitated by the 

 interradial passages and the corresponding series of spaces in vacuolate 

 protoplasm." 



Of the nucleus he says: 



" The liner phenomena of caryokinesis display to us, in a manner 

 such as does scarcely any other process, working nature at her loom. 

 Groujis of granules are the raw material which she next arranges in 

 rows of threads. In astonishment we observe the most manifold loop- 

 ing and splitting of the granular threads and the completion of the 

 most delicate chromatin structures." * * * " The structure of the 

 nucleus is variable only during the periods of division. In the condi- 

 tion of rest its structure * * * is monotonous. 



"The function of the nucdeus can only be such an one as is entirely 

 independent of the differentiation of protoplasm [in diflerent species of 

 living forms], such an one, indeed, as is needed by the most diverse pro- 

 toplasmic structures. This function can only be trophic. * * * 



"Whether this trophic or directive function relates to the metabolic 

 processes, lio the formation of centers, or to the regulation of diffusion- 

 streams of protoplasm, must remain undeci<led, as much as the proba- 

 bilities are in favor of the latter. 



"The essential nature of the structure of the nucleus is difficult to 

 make out; the fundamental form seems to be a radial one, and in such 

 cases may usually be referred to a radial structure, which is not directly 

 manifest externally. 



S. Mis. 4G 3G 



