A Comparative Study of the Structure and Origin of the Yolk Nucleus. 709 
rings. These rings correspond on the one hand to the rings observed in 
leucocytes and in Ascaris and on the other hand, correspond to the 
concentric layers of yellow and white yolk in the bird’s egg, the central 
part being the “Anlage” of the latebra in bird’s eggs. 
15. Rejecting all real nuc-lei and amorphons masses of yolk granules 
and metaplasm, and confining the term yolk nucleus to the centrosphere 
like that of spiders first described under that name, we have to conclude 
that this vitelline body is derived frorn the centrosome of the dividing 
oogonia. Only indirectly as food can metaplasm be said to take part in 
its formation. 
16. Since it is often visible as a single body in late stages of the grow- 
ing oocyte, it affords evidence of persistence of the centrosome for several 
years in some eggs. It affords evidence of structure in the cytoplasm 
which together with the germinal vesicle, causes a polarity in the egg, 
which presumably cannot be ascribed to Chemical action, nor to the effect 
of gravitv or other external influences; for it determines the vegetative 
pole of the egg, since it is the center of growth, and eonsequently the center 
around which the greatest amount of yolk is deposited. 
17. As regards its origin de novo, it shares the fate of the centrosome, 
but affords evidence of the permanenee of that body as a cell organ. The 
origin of centrosome de novo has not yet been proven. Published aceounts 
of the disappearances of centrosomes are being discredited. Disappearance 
of such a body in a mass of yolk granules need not mean anihilation by 
any means. 
18. The yolk nucleus as defined (vitelline body) does not arise from 
extruded chromatin, nor from migrating nucleoh, nor from leucocytes 
or devoured cells. It is the morphologieal center as it is the physiological 
center of the cytoplasm. It may be a center of low oxidation and a center 
of fermentation since it is in it that the karyolymph usually does its 
work of synthesis, which is suggested by the origin of metaplasm in its 
vic-inity. 
19. Its many stränge forms shown in the plates are due : (1) to inereas- 
ing amounts of yolkgranules in its neighborhood ; (2) to the formation of 
vacuoles and the resulting compression of concentric zones of which it 
typically consists; (3) to the variable state of tension or relaxation of 
the astral rays, which becorne conspicuous when aggregated, but incon- 
spicuous when at rest, like the cytoreticulum with which it is continuous. 
20. As the macronucleus in Infusoria gives the staining reaction of 
chromatin, the yolk nucleus of eggs cannot very well be homologized 
with that body as Henneguy and Julin have done. 
