A Comparative Study of the Structure and Origin of the Yolk Nucleus. 695 
appearance and performing the function of a PuRKiNJEan vesiele), and 
the whole becoming enveloped in a Shell, is, however, the ordinary, and 
only method of egg development in many lower animals. In the Trema- 
todes, Cestodes, and the great number of the Turbellaria, the yolk and 
the PuRKiNJEan vesiele are formed in two separate Organs.” 
Speaking of insect eggs, Lubbock (56) says: “We cannot, therefore, 
dass as false eggs those which arise from more than one cell. Perhaps 
it would be better to distinguish the two classes as compound and simple 
or unicellular.” 
Prenant (73) says of Plathelminthes : “In flat worms, the Separa- 
tion of the nutritive and the formative vitellus is still more narrowed. 
With them, the nutritive yolk belongs exclusively to special cells — 
vitelline cells furnished bv one particular gland, the vitellogene. These 
cells arrange themselves around the formative cell, the germigene. Thus 
results a composite egg.” 
Wilson (95) says of hydroids that the egg may actually feed upon 
surrounding cells, taking them bodily into its substance, or fusing with 
them, and assimilating their substance with its own. In such cases the 
nuc-lei of the food cells long persist in the egg cytoplasm, forming the 
socalled pseudocells, but finally degenerate, and are absorbed by the egg. 
It would here seem as if a struggle for existence took place among the 
young ovarian cells, the victorious individuals persisting as the egg; 
and this view is probably applicable, also, to the more usual case, when 
the egg is only indirectly nourished by its brethren. He cites Doflein (23) 
in support of this. A similar generalization as to a struggle for existence 
among germ cells has been expressed by Munson (64), based on numerous 
observations on both egg and sperm cells. He refers, however, to cases 
where this struggle results in the elimination of the germ cells. This seems 
to be a common phenomenon. 
It seems that in some cases cells entering the egg cytoplasm may 
persist for some time as the test cells of truncates, where there is little 
room for doubt in regard to their origin. Wilson (95) refers to the observa- 
tions of Floderus(26) inAscidians, and to Obst’s observations, of similar 
import, in eggs of Molluscs. 
Early writers like Lubbock did not regard the fertilized egg as a single 
cell, and where several cells fuse and persist in the egg, he would admit 
the propriety of speaking of compound eggs. Yet he says of Myriapods, 
the eggs of Litholius , Cryptopus, Geophilus, Arthronomalus, Polydesmus 
and Julus are simple, the vitelline vesiele occurring in some of them being 
probably homologous with the yolk nucleus of spiders. He apparently 
