714 OF REPRODUCTION. 



( 938) ; 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 ani- 

 mals ; and the Development of Man, of a Bird, of a Reptile, or of a Fish, 

 takes place, up to a certain point, upon the same general plan. 



936. The Embryonic Cell, and the cluster of cells that surrounds it, hav- 

 ing arrived on the surface of the Yolk by the movement just described, con- 

 stitute 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 (Plate I., Fig. 16, b) of the Embryonic Cell, now 

 become elongated, and itself beginning to be developed into cells. The same 

 process then takes place within the Embryonic Cell, which has been described 

 as occurring within the Germinal Vesicle ( 916); the granules forming the 

 periphery of the nucleus are first developed into cells, and these are pushed 

 outwards by a new series subsequently generated near the centre. From the 

 mass of cells thus formed, a hollow process passes down into the Yolk; and 

 this gradually extends itself in the same manner as did that formed from the 

 Mulberry-mass, until it includes the whole Yolk, and comes into contact with 

 the inner surface of the layer of cells already mentioned as forming the serous 

 or external lamina of the Germinal Membrane. This second layer of cells 

 is probably that which forms the vascular lamina of the Germinal Membrane. 

 A third process seems to be afterwards sent down, from a part of the nucleus 

 somewhat interior to that from \vhich the last proceeded: and this becomes 

 the mucous or internal lamina of the Germinal Membrane. 



937. The cell-germs forming the periphery of the Nucleus having been thus 

 developed, those nearer the centre then begin to exhibit a corresponding acti- 

 vity. Their evolution follows exactly the same plan as that which has been 

 described in regard to the contents of the Germinal Vesicle ( 916); with 

 the exception that these are arranged in an elongated and not in a circular 

 form. The shape of the nucleus at this time may be compared to that of a 

 pear; the large end marking the situation of the Head ; whilst the prolonged 

 portion is the rudiment of the body. On the median line is seen a groove, 

 occupying the situation in which the Nervous Centres are to be subsequently 

 evolved (Fig. 25, Plate II.). These, when first developed, are surrounded by 

 a tubular structure, which has but a temporary existence in the higher Verte- 

 brata, but which is permanent in the lower Fishes : this structure, termed the 

 Chorda JJoraalis, is found, wherever it exists, to be entirely composed of 

 nucleated cells. From the cells which are exterior to these, is produced the 

 Vertebral Column ; and the mode in which this originates is somewhat as 

 follows. The cells on cither side of the central ^pace (in which the elements 

 of the nervous system are not yet developed) rise up in a ridge, so that the 

 central space becomes a groove ; these two ridges gradually rise up and ap- 

 proach one another, and they are then observed to contain, in what subse- 

 quently becomes the thoracic region, a few pairs of small opaque plates. 

 The ridges (termed plicifi dorsalcs,or dorsal lamina?) continue inclining towards 

 each other, until they coalesce, so that a complete tube is formed ; and in this 



