A Comparative Study of the Structure and Origin of the Yolk Nucleus. 689 
Lubbock (56), studying principally the myriapods, refers to the 
constant presenee of the yolk nucleus as one reason for assigning to it an 
important funetion, though, on the whole, he does not attach to it much 
importance. 
In this matter as in many of his other observations, Balbiani (4) 
is suggestive. Thus, he admits seeing the yolk nucleus in the egg of the 
dog, of the cat, of the squirrel, of the cow, and in the human ovum; but 
he says, the study is diffieult because its refraction is the same as that 
of the vitellus. It is necessarv, he says, to examine young follicles, where 
the vitellus is still homogeneous and transparent; and then he warns us 
not to use any reagents that will affect the transparency of the vitellus. 
The failure to find the yolk nucleus in many cases, can be attributed 
first to the fact that, with the exception of Munson (61), no real, systematic 
and persistent attempt has been made to trace the historv of the body 
in a single egg, haphazard observations being reliecl on to reveal its presenee 
in as many different eggs as possible. The historv of the yolk nucleus 
resembles the history of biological Science, in that it has passed through 
a natural history stage of mere seeing and naming. A second reason for 
failure is as stated by Balbiani, that in transparent protoplasm it too 
is transparent; and in opaque protoplasm rendered so by reagents, it, too, 
is equally affected leaving it as invisible as ever. 
This ought to afford a warning not to attach much importance to 
Statements as to the absence of this body. For while the investigator’s 
inability to adapt his methods to the subjec-t may thus be indieated, no 
positive proof is given of the absence of such a structure. 
In the literature, the following cytoplasmic inclusions have been 
called yolk nuclei: 1. a single large spherieal body, with fibrous capside 
enclosing a vesicle filled with granules or with a more or less transparent 
substance; 2. several, scattered, small bodies resembling ordinary cell- 
nuclei in shape, size, and staining reaction; 3. amorphous masses of stain- 
able granules encirchng the nucleus; 4. scattered masses of stainable 
substance supposed to be more or less fluid; 5. single, definite masses 
either spherieal or crescent-shaped, resembling arc-hoplasm, closely 
applied to the germinal vesicle, or removed from it so as to occupy 
approximately the center of the egg; 6. definite attraction spheres, with 
astral rays and centrosome. 
It is diffieult to believe consequently that Lerboullet (51), Cramer 
(20) and Reichenbach (76), though often quoted, in this connection, 
have described the real yolk nucleus. The latter, speaking of developing 
c-rayfish eggs, showing Segmentation nuclei at the peripherv says: “In 
