A Comparative Study of the Structure and Origin of the Yolk Nucleus. 667 
one third the size of the Purkin JE an vesicle, may generally be seen in it. 
It soon disappears and is replaccd by a patch resembling tliat in Julus 
In the following year, Lerboullet (51) described a body in the cyto- 
plasm of eggs of crayfish. A radial arrangement of the yolk surro unding 
the body is suggestive in view of our present knowledge on tliat subject. 
The impetus wliich Gegenbaur had given to the study of the egg, 
and the problems which that study had already created, led Balbiani (2) 
to make a comparative study of the eggs of different animals. He claimed 
to have found the body in Helix, and in representatives of most classes 
of animals, while in some species, he found no trace of it. He seems to 
have studied the living egg. Of Tegeneria he says: “At the moment wlien 
it becomes visible in the youngest eggs, it has the appearance of a httle 
homogeneous and transparent vesicle, placed between the germinal vesicle 
and the egg stalk. It is at first much smaller than the germinal vesicle ; 
but as it grows faster, it soon equals it in size. It consists of a great number 
of concentric layers forming a sort of capsiüe around the nucleus (central 
vesicle?), whose refringent aspect separates it distinctly from the cyto- 
plasm of the young egg, which is still transparent. When the outer lam- 
inated layer is ruptured, the vesicle in the interior is found. This vesicle 
contains a pale and granulär substance in which is seen a round capsule. 
In some forms of spiders, the laminated capsule is replaccd by homo- 
geneous or granulär substance enclosing a central vesicle.” 
In 1872, Eimer (25) saw in the center of the egg of lizard, a spherical 
body which he regarded as the “Dotterkern”, and which he considered 
to be a constant element in the egg of reptiles. 
Kolessnikoff (44) mentioned 1878 granulär yolk nuclei in egg of 
several species of frogs and in egg of the toad. 
He figured the yolk nucleus in eggs of fishes and Bambeke (6) has 
also seen it in fish eggs. 
In 1877, the yolk nucleus was again described in the eggs of crayfish 
by Reichenbach (76). It is said to consist of a central body with radial 
arrangement of the yolk granules. 
In 1882, J. Schütz (82), in his Inaugural Dissertation, gave special 
attention to the yolk nucleus. His plates show many figures of it. It is 
represented cliiefly as a spherical body, if I remember correctly. It is 
now some years since I saw his paper. I cannot very well describe it 
from memory. Recentlv I have not been able to secure a copy of it. 
In 1822, also, Jatta (38) described a peculiar yolk nucleus in Pholcus 
phalangioides — • “H nucleo vitellino omogeneo, allungato, in arcato in- 
torno alla vesicola germinativa e colarato intensamente in rosso.” 
Archiv f. Zellforschung. VUI. 
44 
