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 mesogloea ; 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 archieocytes." 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 archoeocytes 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 arclucocytes, and, therefore, also to the 



