A Comparative Study of tke Structure and Origin of the Yolk Nucleus. 691 
According to Carus (18), it is the center of formation of the plastic 
substance of the egg, while the nutritive parts seem to collect around the 
germinal vesicle. The granulär body seen by Cramer (20) in the young 
transparent egg of the frog, and also seen by Carus, was compared by 
them to the yolk nucleus of Arachnids, observed by von Wittich (97); 
and was thought to form on its surface the vitelline granules. Balbiani (3) 
also said that it is around this body, whenever it is to be seen, that the 
granules of the cytoplasm, which constitute the plastic part of the egg, 
or the germ, are formed. 
Lubbock (55) regarded it as a Condensed portion of the cytoplasm. 
In myriapods, he described it as a clear vesicle, often enclosed by a circle 
of little, bright granules, which are sometimes scattered throughout the 
vesicle. The protoplasm surrounding it becomes Condensed, and in it, 
granules are formed, which spread throughout the cytoplasm, and forms 
on the periphery of the egg a continuous layer which constitutes the germ. 
In both vertebrates and invertebrates, the yolk nucleus has been 
described as a clear vesicle, varving in size, and surrounded by a zone 
of concentric fibers, or by a zone of granules or both. In the frog’s egg, 
Reichenbach (76) found that the central body is surrounded by a radial 
arrangement of the yolk, which is of special interest in view of the fact 
that the centrosome and aster had not yet been discovered. 
The central vesicle is often called a nucleus. Thus Henneguy (32), 
quoting Balbiani, says of this body in the frog. “It is always a granulär 
mass, containing a little clear nucleated vesicle which is the nucleus.” 
Says Balbiani (3): “La vesicule embrvogene est une cellule, puisqu’elle 
est formee d’une masse du protoplasma, avec un noyau et un nucleole. 
Ces deux derniers elements ne sont generalement pas difficiles ä apercevoir, 
mais le protoplasma se confond souvent avec celui de l’oeuf parsqu’il 
possede la meme refringence.” 
Because of its similarity to the nucleus of an ordinary cell, a similarity 
especially striking in many cases where there are several scattered through- 
out the cytoplasm, it has been surmised that it results from cells wander- 
ing in either as follicle cells or as leucocytes. Such appearances of which 
the ascidian test cells are good examples, led many reliable observers 
to doubt the correctness of Gegenbaur’s generalization that all eggs 
are single cells. 
At this stage of development of the subject, the problem of the 
yolk nucleus becomes intimately involved in the problem of nutrition of 
the egg, and its great accumulation of yolk as compared with ordinary 
cells. At the same time, also, the discovery of the phenomena of partheno- 
