RECENT RESEARCHES ON OOGENESIS. 603 
cells within, but near the periphery of the ovum. Some 
observations, such as those of Schafer and Bambeke, may also 
be interpreted in harmony with Fol’s theory, but it was not 
till 1883 that attention was emphatically recalled to the 
subject. 
Following up his previous research, Fol distinguished in the 
ovum of Tunicata (1) granular globules encrusted on the 
vitellus ; (2) a thin “chorion” membrane; (3) a layer of 
papillary “ spumeuses” cells; (4) an outermost envelope of 
flattened pavement-like follicular cells. Only the two outer 
layers are distinctly cellular, and result from a migration out- 
wards of cells formed endogenously within the ovum, and with 
the distinct participation of the germinal vesicle. The endo- 
genous formation begins by a local thickening of the nuclear 
membrane, the thickened portion is pressed outwards, and the 
nucleolus, which is generally near, seems to yield a little of its 
substance to the protrusion. The latter soon becomes a solid 
button-like bud, is attached to the nucleus by a distinct stalk, 
but being liberated migrates thence outwards, and forms, with 
associated protoplasm, a follicular cell. Fol describes these 
budded daughter-nuclei as appearing sometimes in succession, 
and then the nucleolus of the germinal vesicle remains visible, 
or as being formed in numbers at once with the disappearance 
of the nucleolus. The first set of migrant nuclei form the 
thin flat outermost layer, the so-called papillary cells are next 
formed, while a third set of migrating elements, not, however, 
true cells nor arising from the germinal vesicle, form the 
granular globules or so-called test-cells. 
Sabatier, on the other hand, maintained that though the 
follicular cells of Tunicata were intravitelline and not from 
outside, yet they arose from aggregations of chromatin in the 
vitellus, near the germinal vesicle, but not from it. He de- 
scribes them as protoplasmic concentrations, at first clear and 
homogeneous, but becoming differentiated into cells with dis- 
tinct nuclei. The granular or “test” cells are also intra- 
vitelline eliminations, but they degenerate before their cellular 
differentiation is accomplished. Between follicular cells and 
