990 OF GENERATION. 



more opaque circle (whose opacity is due to the greater accumulation of cells 

 and nuclei in that part than in the area pellucida), which subsequently becomes 

 the area vasculosa. In the formation of these two spaces, both the serous and 

 mucous layers of the germinal membrane seem to take their share ; but the 

 foundation of the embryonic structure, known as the primitive trace, is laid in 

 the serous lamina only. This consists in a shallow groove (c), lying between 

 two oval masses (6), known as the laminae dorsales. The form of these 

 changes with that of the area pellucida ; at first they are oval, then pyriform, 

 and at last become of a guitar-shape. At the same time, they rise more 

 and more from the surface of the area pellucida, so as to form two ridges of 

 higher elevation, with a deeper groove between them ; and the summits of these 

 ridges tend to approach each other, and gradually unite, so as to convert the 

 groove into a tube. At the same time, the anterior portion of the groove 

 dilates into three recesses or vesicles (Plate II. Fig. 12, 6), which indicate the 

 position of the three principal divisions of the Encephalon, afterwards to Tbe de- 

 veloped as iheprosencephalon, the mesencephalon, and the epencephalon ( 1012). 

 The most internal parts of these laminae, bounding the bottom and sides of the 

 groove, appear to furnish the rudiments of the nervous centres which this cranio- 

 vertebral canal is to contain ; whilst the outer parts are developed into the rudi- 

 ments of the vertebral column and cranium. Even before the laminae dorsales 

 have closed over the primitive groove, a few square-shaped and at first indistinct 

 plates, which are the rudiments of vertebrae (c), begin to appear at about the 

 middle of each. The position of the bodies of the vertebrae is indicated at this 

 period, in the embryos of Birds and Fishes, by a distinct cylindrical rod of 

 nucleated cells, termed the chorda dorsalis; and this retains its embryonic type 

 in the Myxinoid Fishes ( 1010). While this is going on, an accumulation of 

 cells takes place between the two laminae of the germinal membrane at the "area 

 vasculosa ;" and these cells speedily form themselves into a distinct layer, the 

 vascular lamina, in which the first bloodvessels of the embryo are developed, 

 as will be presently described ( 998). From the dorsal laminae 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 processes 

 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 

 inclose the spinal cord. At the same time, the layers of the germinal mem- 

 brane which lie beyond the extremities of the embryo^ are folded in, so as to 

 make a depression on the yelk ; and their folded margins gradually approach one 

 another under the abdomen. The first rudiment of the intestinal canal presents 

 itself as a channel along the under surface of the embryonic mass, formed by 

 the rising up of the inner layer of the germinal membrane into a ridge on either 

 side. The two ridges gradually arch over and meet, so as to form a tube, which 

 is thus (so to speak) pinched off the general vitelline sac ; and it remains in 

 connection with this, by means of an unclosed portion, which constitutes the 

 " vitelline duct" (Figs. 262, 263). 



998. Whilst these new structures are being produced, a very remarkable 

 change is taking place in tnat part of the serous lamina which surrounds the 

 area pellucida. This rises up on either side in two folds; and these gradually 

 approach one another, at last meeting in the space between the general envelop 

 and the embryo, and thus forming an additional investment to the latter. As 

 each fold contains two layers of membrane, a double envelop is thus formed ; 

 of this, the outer lamina adheres to the general envelop ; whilst the inner re- 

 mains as a distinct sac, to which the name of Amnion is given. (See Figs. 264, 

 265, and 266.) This takes place during the third day in the Chick; the period 

 at which it occurs in the Human Ovum is difficult to be ascertained, owing to 

 the small number of normal specimens which have come under observation at a 



