164 E, RAY LANKESTER, 
and other Mollusca, have shown that the residual yelk does 
contain formative and not purely nutritive material. It isin 
fact, in function, as well as morphologically, the equivalent of 
the invaginated cells of embolic Planule. The structure- 
forming protoplasm is there, but it isso enormously distended 
with nutritive granules or molecules that it often cannot 
cleave at all, sometimes to but a limited extent, and then, by 
a late segregation similar to that by which it gave rise to the 
first cap of formative protoplasm, it sets free at one or at 
many points the corpuscular elements which constitute the 
tissues derived from endoderm or hypoblast. 
The term “ food-material” or “ nutritive material” I propose 
to apply to those granules and molecules (the deutoplasm of 
Van Beneden) with which the protoplasm of the residual 
yelk is gorged, as an Ameeba may be gorged with solid par- 
ticles of food. 
- Without entering into further explanations of terms I 
subjoin a condensed statement of some of the important 
variations presented by Planule. 
A. The endoderm is separated by fission from the inner 
surface of a series of cells which form a hollow polyplast or 
blastosphere— 
= DELAMINATE PLANULA. 
These are at present known only in the Hydrozoa and the 
Calcareous Sponges (?). 
B. The cells of the polyplast or blastosphere (mulberry- 
form, morula) do not individually divide each into an ecto- 
dermal and an endodermal cell, this separation having been 
already effected in the first cleavage of the egg. ‘The single- 
layered blastosphere or polyplast consists of two hemispheres 
or areas constituted respectively by the future ectodermal cells 
and by the future endodermal cells (or their parents). By dif- 
ference in direction of growth the latter becomes enclosed 
within the former— 
=INVAGINATE PLANULZ&. 
These latter may present, as extreme forms of one and the 
same process, connected by intermediate forms— 
1. Relatively little or no difference in the amount of food- 
material incorporated in the cells of the ectodermal and the 
cells of the endodermal area of the polyplast, and accordingly 
an equality in the size and activity of those two sets of cells. 
In this case the endodermal cells become tucked or pushed 
into the cavity (segmentation cavity) of the polyplast, form- 
ing a follicular depression (the primitive gastric cavity), 
the margin of which constitutes a hole or orifice called 
