DEVELOPMENT OF THE EMBRYO. 611 



bryonic mass, and of the membranes surrounding the Yolk-bag; and then to 

 the origin of the Vertebral column, Digestive organs, and Circulating apparatus. 



758. The Ovum, when it quits the Ovariurn, has been stated to contain 

 within the Germinal Vesicle, two cells which did not exist there previously to 

 fecundation : and from each of these, two new cells are subsequently produced, 

 which in their turn give birth to eight others ( 745). In this manner, the 

 number of vesicles originating in the twin-cells of the Germ is continually 

 increased, until at last they become too numerous to be counted, and form a 

 cluster resembling a Mulberry in appearance; this mulberry-like structure 

 may be conveniently termed the Germinal Mass (Plate I. Fig. 15, a). In the 

 centre of this mass there is found a peculiar Cell, differing from the rest in its 

 greater size, and in possessing a very well defined annular nucleus, with a pel- 

 lucid cavity in its centre (Fig. 16, a, b). From this peculiar Cell, all the parts 

 which enter permanently into the composition of the Embryo are developed ; 

 the vesicles forming the exterior of the germinal mass being subservient to a 

 merely temporary purpose. This central or Embryonic Cell is gradually 

 brought to the surface of the Germinal Mass, by the formation of a cavity (c) 

 in the interior of the latter ; for the layer of cells within which this cavity is 

 formed, progressively extends itself, until it comes into contact with the inner 

 surface of the Yolk-bag, having absorbed the yolk into the hollow thus left. 

 Thus out of the periphery of the Mulberry-mass, appears to be formed the 

 exterior layer of what is termed the Germinal Membrane; this membrane is 

 first seen as an epithelium-like layer of cells, covering the Yolk; but beneath 

 this layer, which is afterwards known as the serous lamina of the Germinal 

 membrane, two others are subsequently produced from the central portion of 

 the Germinal mass. Now it is highly interesting to observe, that this Ger- 

 minal Membrane, which in the higher animals is a mere temporary structure, 

 subservient only to a temporary function, forms, in the lower tribes, the greater 

 part of the permanent fabric of the body. Thus, in the Polypes, the cavity in 

 which the Yolks enclosed becomes a Stomach; the external layer of the 

 Germinal Membrane becomes the integument ; whilst the internal forms the 

 lining of the Digestive cavity, of which the mouth is formed by absorption of 

 its wall at one point. Here the Yolk is directly absorbed and assimilated by 

 the surrounding membrane. In the higher Oviparous animals, the Germinal 

 Membrane serves to absorb nutritious matter from the Yolk, and to prepare it 

 for the use of the Embryo itself, by converting it into Biood ( 762) ; but, after 

 the Yolk has been exhausted, the Yolk-bag is taken into the body, and is 

 gradually removed by absorption. In Mammalia, these structures are of less 

 importance. The store of Yolk, laid up for the nutrition of the Embryo, is 

 comparatively inconsiderable ; being only destined to serve for the short time 

 that elapses, before the Ovum forms its new connection with the Parent, 

 through the medium of the Chorion; and the Yolk-bag is ultimately separated 

 from the Embryo, and thrown off as useless. Still the early processes are the 

 same in Mammiferous, as they are in Oviparous animals ; and the Develop- 

 ment of Man, of a Bird, of a Reptile, or of a Fish, takes place, up to a certain 

 point, upon the same general plan. 



759. The Embryonic Cell, and the cluster of cells that surrounds it, having 

 arrived on the surface of the Yolk by the movement just described, constitute 

 what is known in the Bird's egg under the name of the Cicatricula. This is 

 a semi-opaque disc, composed of numerous flattened cells ; and in the midst 

 of it is seen a round transparent space, termed the Area Pellucida, which is 

 nothing else than the place occupied by the large Embryonic Cell, now become 

 flattened, and still retaining its clearness. In the centre of this is seen a very 

 faint line, which is termed the Primitive Trace; and this is the large annular 

 Nucleus (Fig. 16, b) of the Embryonic Cell, now become elongated, and itself 



