612 OF REPRODUCTION. 



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 ( 745) ; the granules forming the periphery of the nucleus 

 are first developed into cells, and these are pushed outwards by a new series 

 subsequently generated nearer the centre. From the mass of cells thus formed, 

 a hollow process passes down into the Yolk ; and this gradually extends itself, 

 in the s,ame manner as did that formed from the Mulberry-mass, until it includes 

 the whole Yolk, and cornes into contact with the inner surface of the layer of 

 cells already mentioned as forming the serous or external lamina of the Ger- 

 minal 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 which the last proceeded ; and this becomes the mucous or internal 

 lamina of the Germinal Membrane. 



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

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

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

 been described in regard to the contents of the Germinal Vesicle ( 745) ; 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 Ver- 

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

 the Chorda Dorsalis, 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 either side of the central space (in v^hich 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 

 approach one another, and they are then observed to contain, in what sub- 

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

 The ridges (termed plicae dorsales, or dorsal laminae) continue inclining towards 

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

 tube an indication is soon perceived of a division into vertebra?, of which the 

 plates just mentioned are the incipient arches (Fig. 26, Plate II.). Towards 

 the anterior extremity, however, the dorsal laminae do not at once close in ; 

 and the large cells, in which the great divisions of the Encephalon originate 

 ( 214), may be seen between them. From the Dorsal Lamina on either side, 

 a prolongation passes outwards and then downwards, forming what is known 

 as the ventral lamina; in this are developed the Ribs and the transverse pro- 

 cesses of the Vertebrae ; and the two have the same tendency to meet on the 

 median line, and thus to close in the abdominal cavity, which the dorsal 

 laminae have to enclose the spinal cord. At the same time the layers of the 

 Germinal Membrane, which lie beyond the extremities of the Embryo, are 

 folded in, so as to make a depression on the yolk ; and their folded margins 

 gradually approach one another under the abdomen. In these two modes, a 

 cavity is formed beneath the Embryonic mass, which is separated from the 

 general cavity of the Yolk by the folds just described ; but these still leave a 

 passage which, in the Bird, remains of considerable size until a much later 

 period, but which, in the Mammiferous Ovum, is soon obliterated. For the sac 

 which contains the yolk, and from which the abdominal cavity is pinched off (as 

 it were) at a very early period, is destined, in the Mammiferous animal, to be 



