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peculiar protuberance, which CRAmPTon!) has, apparently rightly, 
homologized with the Molluscan yolk-lobe, and WıLson states that 
QUATREFAGES has described a similar protuberance in the ova of Sabella 
(Hermella). 
The idea as to the significance of the yolk-lobe which most 
naturally suggests itself is that it is a mass of food material either 
destitute of cytoplasm or else unacted upon by the tensions produced 
in the cytoplasm during karyokinesis. It is difficult to understand 
however, on such an hypothesis, why Fulgur with such a very large 
amount of yolk and Chaetopterus with so little should both possess 
minute lobes, while in Nassa and Ilyanassa with relatively moderate 
amounts of yolk they are so large; and furthermore this idea is opposed 
by the fact that in Chaetopterus the astral ale during karyokinesis 
extend into the yolk-lobe (Mrap). 
In Ilyanassa CramprTon has discovered an interesting relation of 
the yolk-lobe, which does not, however, explain its occurrence. He 
has shown that it is attached to the macromere from which the 
mesoblast pole-cell arises, and that when it is removed from a 
developing egg, the resulting embryo shows a “complete absence of 
cells corresponding in position to the mesoblast bands”. The conclusion 
is drawn from this that the presence of the yolk-lobe determines the 
formation of the mesoderm pole-cell, but though this conclusion is 
fairly deducible from the observations on Ilyanassa it cannot be 
considered a general law, since there are many Annelid and Molluscan 
forms (Nereis, Amphitrite, Crepidula, Umbrella, Unio etc.) in which 
the macromere which produces the mesoderm pole-cell possesses no 
yolk-lobe. 
The somewhat sporadic occurrence of a yolk-lobe, however, would 
seem to deprive it of any great phylogenetic or cytogenetic significance, 
and it presumably owes its formation to some peculiar physical 
conditions existing in certain ova. What these conditions may be 
and why such forms as Ilyanassa and Chaetopterus possess a yolk- 
lobe, while Crepidula and Arenicola have none, are problems for future 
study. 
In all the ova examined the nuclei were undergoing karyokinesis 
preparatory to the formation of the four-cell stage, and though the 
entire process of karyokinesis was not observed, yet several interesting 
1) H. E. Crampron, Experimental Studies in Gasteropod Development. 
Arch, f. Entwickelungsmech. der Organismen, Bd. 3, 1896. 
