DIFFERENT KINDS OF CELLS. 17 



At first, these globules are grouped towards the periphery 

 of the cavity of the primitive (Fig. 3) ovum, and, in this 

 way, form a membrane which we shall study under the name 

 of epithelium. As in the perfected organism, an epithelium 

 rests upon a fibrous or undetermined tissue, so also does the 

 ovular epithelium rest upon the 

 membrana pellucida (Fig. 3, A). 

 We see then even at this stage 

 (and great importance must be 

 attached to these forms) the organ- 

 ism represented successively by a 

 cell, and secondly by an epithelium ; 

 this latter might be called epithe- 

 lium of the zona pellucida (Fig. 3 B) ; 

 and as this serves as the germ of , , 



,, A . T i* Diagram of the Blastoderm.* 



all the other portions it has been 



called the germ-membrane, or, more generally, blastodermio 



membrane, or blastoderm. 



This change of position of the globules, whence a globular 

 membrane results, is soon followed by a change of form, 

 whence there occurs a separation into distinct layers in this 

 membrane ; take for example one of the meridians of the 

 blastoderm where the globules become multiplied more than 

 in any other place ; here the blastoderm, as with every epi- 

 thelium which becomes hypertrophied in a certain portion, 

 is obliged, as will be seen later in the formation of glands 

 and papilla, to swell out and form a sort of pouch on which 

 may be lodged the new globules that are formed. This 

 pouch or villosity (Fig. 3 C) is the first rudiment of the 

 embryonic body. Without now going into the details, it is 

 necessary simply to mention that at this point the globules 

 become separated into three layers or folds, viz., the external, 

 internal, and intermediate folds. 



The external fold, called the corneal, maintains its glob- 

 ular condition, and from this is formed our epiderm, our 

 external cuticle, 1 and such of the organs as may be derived 



1 A comparison of the two kingdoms demonstrates the fact 

 that both animals and vegetables have an external envelope, com- 

 posed of analogous cells; so that we can apply to each the name 

 cuticle. Yet in the vegetable the cuticle is very simple, and almost 

 everywhere the same, but in the animal it is complex, and, accord- 



* A, Vitelline membrane. B, Simple form of Blastoderm. C. Point where 

 the blastoderm is already composed of three layers of cells, three folds. 



2 



