244 CEAVPOLE. [VOL. XIV. 
Passing now to the early changes in the nutritive cells, a 
consideration of that peculiar structure known as the yolk 
nucleus is necessary. 
The “yolk nucleus” is a term of varied application and, 
consequently, great indefiniteness. Two distinct classes of 
structures have been designated by this name: (1) embryonic 
cells concerned with yolk absorption whose origin is assigned 
variously to entoderm, mesoderm, superfluous spermatozoa, or 
the germinal vesicle, and whose fate is said to be absorption 
or transformation into the midgut epithelium; (2) all those 
structures appearing in the ovarian egg which can have but 
one of two possible origins: nucleus or cytoplasm of the cell 
itself, and whose existence may cease before the yolk appears 
in the egg or may be continued into embryonic life. 
It is on the second of these classes that the facts observed 
in Anurida have a bearing. The peculiar “blue caps” that 
appear at a certain stage in the nutritive cells can have no 
other homology than the “yolk nucleus,” the “ Dotterkern”’ 
or ‘“Nebenkern”’ of some authors. 
Hubbard ('94) gives a list of groups in which the yolk nucleus 
has been observed to occur. This list includes all the classes 
of arthropods, cnidarians, nematodes, Sagitta, lamellibranchs, 
gasteropods, and all the vertebrate groups. The nucleus is 
found in eggs associated with nutritive cells and those without. 
Stuhlmann (’86) described it in Bombus, Vespa, Trogus, Pimpla, 
and Bauchus, varying in form from a diffuse peripheral mass 
toa localized spot. He, however, reduces the method of origin 
in all the Hymenoptera to one type, —that of a small concre- 
tion appearing close to or in the near vicinity of the germinal 
vesicle. This mass wanders away, forms a peripheral layer, 
collects at one pole or is scattered in several diffuse masses. 
Stuhlmann never satisfied himself of its nuclear origin as Bal- 
biani (83) had in Geophilus and Will ('s4) in the frog. In all 
cases it is in very young ova that it appears; it becomes in- 
visible on the formation of yolk. No mention has been made 
of the existence of the yolk nucleus in any other of the forms 
of insects. The appearance in Anurida of this blue cap in the 
nutritive cells declares the presence of this curious adjunct of 
