A Comparative Study of the Structure and Origin of the Yolk Nucleus. 683 
process, or whether tliey arise from fragmentation of the germinal vesiele, 
would be a very interesting subject for investigation. But there is no 
reason to suspect that they have anything to do with the formation of 
the vitelline bodv. Their occasional presenc-e in what at first sight seems 
to be normal eggs, was to me a very perplexing matter, tili I was able to 
convince myself, that even where the c-ytoplasm seems normal, their 
presence and the absence of the germinal vesiele, is evidence of beginning 
degeneration. The whole matter, as far as the question of this origin of 
the yolk nueleus is concerned, is put in a new light, as soon as later stages 
of degeneration, when these real nuclei are present, ean be examined. 
While the central vesiele, both in the spider and in Limulus and also 
in the tortoise, has the general appearance of a nucleus, it never gives the 
specific chromatin reaction that these nuclei give. The same may be said 
of the other bodies, as tliose in figs. 31, 35, 41, 45, which while they often 
look like nuclei, never give the specific chromatin reaction. I have, there- 
fore, called them vacuoles; and the graniües which they contain, I have 
regarded as metaplasm, — a form of food material which may be stored 
as reserve food, giving rise to yolk; or, as oceasion demands, may be again 
absorbed bv the living, growing substance. 
Yolk Nucleus in Egg of Pigeon. 
If the yolk nucleus in the egg of the tortoise is what 1 have intimated 
we shoiüd expect to find something similar in the egg of the pigeon, repre- 
senting the birds, which are so closely allied to the reptiles. It is interest- 
ing to find that this relationship, which has been based on somatic char- 
ac-ters, proves to be equallv dose when the ovarian eggs of the two forms 
are compared. The Statement may be made also regarding the egg of 
Limulus and that of the spider. There is a similarity in the cytoplasm, 
when seen tlirough the mic-roscope, as unmistakable, one is tempted to 
say, as that which a naked eye examination of the bodies of the two 
ereatures reveals. 
In fig. 51, is represented an oogonium of the pigeon’s ovary, a section 
of which is represented in fig. 50. The oogonium is oval. It has a large 
nucleus with distinct chromatin bodies arranged as a network. The cyto- 
plasm is very thin except at one pole of the nucleus. At this point, a 
centrosome and archoplasm spreading out over the nucleus can be seen. 
After some divisions, the oogonia cease dividing; and some of the cells 
are differentiated into follicle cells, surrounding the growing oocyte. 
I have studied this point carefully in Clemmys ; and I have found nothing 
in the pigeon’s ovary that woiüd warrant a different Interpretation. 
Archiv f. Zellforschung. VIII 45 
