THE PRINCIPLES OF EMBRYOLOGY 473 
primitive form, and because its development has been carefully 
worked out recently by Brauer. 
In Hydra we find a dermal layer of cells and an inner layer of 
cells separated by a gelatinous mass known as mesoglcea; in this 
mass between the dermal and inner layers scattered cells are found, 
the interstitial cells, Now, according to Brauer the position of the 
germ in Hydra is the interstitial cell-layer. One cell of the ovarium 
becomes the egg-cell, the others have their substance changed into 
yolk-grains, forming the so-called pseudo-cells, and as such afford 
pabulum to the growing egg-cell. Thus we see that in between the 
dermal and gastral layer of cells a third layer of cells is found, com- 
posed of free living germ-cells, some of which, by the formation of 
yolk-granules, become degraded into pabulum for their more favoured 
kinsfolk. These interstitial cells are said to arise from the dermal 
layer, or ectoderm, but clearly, as in other cases, germ-cells constitute 
a class by themselves and cannot be spoken of as originating from 
ectoderm-cells or from hypoderm-cells. 
So also in Porifera, Minchin states: “In addition to the collared 
cells of the gastral layer, and the various cell-elements of the dermal 
layer, the body-wall contains numerous wandering cells or amcebo- 
cytes, which occur everywhere among the cells and tissues. Though 
lodged principally in the dermal layer, they are not to be regarded 
as belonging to it, but as constituting a distinct class of cells by 
themselves. They are concerned probably with the functions of 
nutrition and excretion, and from them arise the genital products.” 
Further (p. 31): “At certain seasons some of these cells become 
germ-cells; hence the wandering cells and the reproductive cells 
may be included together under the general term archeocytes.’”’ Also 
(p. 51): “ The mesoglcea is the first portion to appear as a structure- 
less layer between the dermal and gastral epithelia, and is probably 
a secretion of the former,” 
He also points out that in these, the very lowest of the Metazoa, 
the separate origin of these archzeocytes can be traced back to a very 
early period of embryonic life. Thus in Clathrina blanca the ovum 
undergoes a regular and total cleavage, resulting in the formation of 
a hollow ciliated blastula of oval form. At one point, the future 
posterior pole of the larva, are a pair of very large granular cells with 
vesicular nuclei, which represent undifferentiated blastomeres and 
are destined to give rise to the archzocytes, and, therefore, also to the 
