592 CHRISTIAN CHAMPY AND H. M. OARLETON 
may be observed in such cells, and it seems definitely to be 
related to variations in surface tension caused by exchanges 
between nucleus and cytoplasm, as has been suggested by 
various authors (e.g. Prenant, 10). 
In other instances, however, the shape of the nucleus, 
notwithstanding its extreme lobulation, is too definite to permit 
of its bemg attributed to surface tension alone. Examples of 
this are the spermatogonia of some Amphibia, in which the 
shape of the nucleus is constant ina given species (Pl. 23, fig. 3). 
Here the nuclear polymorphism is apparently due to the 
intervention of other factors (to be considered later), and 
only such variations from the normal as occur during periods 
of intensive cell-activity —such as growth, differentiation, &¢.— 
can be ascribed to the surface-tension changes that accompany 
such phenomena. Somewhat similar are the modifications 
which occur in many oocytes during development, as shown 
in Pl. 28, fig. 2. In the early stages of differentiation the nucleus 
in such elements is oval, containing one large central nucleolus 
and many smaller peripheral ones. But subsequently the 
nucleus becomes polymorphic, while around it is established 
a clear (endoplasmic) zone in the cytoplasm. Here again do 
we find extreme nuclear lobulation coinciding with enhanced 
metabolism of the cell. 
3. MECHANICAL DEFORMATION OF THE NUCLEUS. 
The study of our material has convinced us that nuclear 
shape is often due to pressure exerted on it by various cell 
inclusions. An obvious example of this is furnished by the 
thin and crescentic nucleus entirely pressed to the periphery 
of the fully developed adipose cell. Somewhat similar is the 
deformation of the nucleus in the duct-cells from the pronephros 
of Triton (see Pl. 23, fig. 4). This is due to the centre of the 
cell bemg occupied by the lumen of the duct. Again, in the 
early segmentation stages of ova containing much yolk, the 
nuclei are indented by the large, nert yolk-dises. Text-fig. 6 
shows such a nucleus from a blastula of the Amphibian 
Triton alpestris. On one side there are deep indentations 
