[101] EMBRYOGRAPHY OF OSSEOUS FISHES. 555 



tween the mcroblastic and the holoblastic types of eggs. The return 

 of the segmentation nucleus towards the center of the egg is apparently 

 prevented by the presence of yelk or deutoplasm in the egg. The second 

 or subsidiary principle qualifying the first in that the degree of recession 

 of tlie segmentation nucleus toicards the center of the egg is determined by 

 the amount of food yelk ivhich is present in its center or at its vegetative 

 pole. 



We must not, however, forget to mention that in the meroblastic eggs 

 of the frog, and Clepsine, according to Whitman, the first segmentation 

 nucleus may return to the center of the egg after impregnation and seg- 

 ment twice, forming four nearly equal cells, before the four new nuclei 

 are finally repelled to the animal pole to establish a meroblostic form of 

 segmentation. These appear to be cases in which the final displace 

 ment of the nuclei has been retarded. The principle involved is the 

 same, however. 



2G. — On some of the phenomena of segmentation and growth. 



Recently the study of the processes of segmentation has been pur 

 sued very successfully by a large number of investigators upon a con- 

 siderable number of forms. One of the most recent papers on the sub- 

 ject is by Prof. A. Rauber,* who has taken special pains to investigate 

 the successive development of the segmentation furrows and their re- 

 lations of direction to each other and to the axis of the nucleus in the 

 act of division, as well as the direction of the cleavage planes of seg- 

 menting cells in relation to the growth in extent of a flat membrane 

 or other structure. He also considers the phenomena of contractility 

 or movement of the protoplasm during segmentation and has arrived 

 at what ai)pear to the writer to be some very important conclusions re- 

 specting what may be regarded as evidence of its structure and the re- 

 lations which this bears to processes of growth and metabolism or waste 

 and repair. Without pretending to review Rauber's important contri- 

 bution to the subject of the morphology of protoplasm, if we may so 

 speak, the writer has for a considerable time past been api)roaching 

 somewhat similar conclusions regarding the nature of the physical sub- 

 stratum of vital phenomena. Many of our most able investigators have 

 been striving to represent it in an altogether too simple and homoge- 

 neous form, until the idea is widely prevalent amongst otherwise well- 

 informed persons that there is really some such thing as an homogene- 

 ous primordial liviug jelly out of which all living beings are formed. It 

 is stated by some to be a colloid, like gelatin or gum, but if we study it 

 narrowly we find that it differs physically in a good many respects from 

 these substances, one of the most striking of which is, that unlike the 

 not-living colloids it will not mix in any given proportion in the living 



* Neue Grundlegunijvn zur Eeniniss do- Zelle. Morphologisches Jahrbuch, Fill, pp. 233- 

 338, pis. XI-XIV. Leipzig, 1882. 



