LECTURES ON EMBRYOLOGY. 



99 



circulation in general. And the lower layer gives 

 rise to the organs by which life is maintained ; in 

 the first place to the various sacs of the alimenta- 

 ry canal, to the various parts of the digestive ap- 

 paratus, the stomach, with its glandular system ; 

 the lung, which is only a sac derived from the 

 alimentary tube, and all the other various glandu- 

 lar organs which are connected with the alimenta- 

 rv canal, and also to the liver, in connection with 

 the blood system, and to the system of reproduc- 

 tion. 



I need not enter into these anatomical details ; 

 but in order to show how this is brought about, let 

 us trace, for a moment, these longitudinal sections, 

 in which the same layer (Plate VIII, fig. E,) is rep- 

 resented in its proportional development. The 

 blue mass represents the upper layer of the germ, 

 as it has grown thicker in the anterior part of the 

 germ. These red masses represent the middle 

 layer (Plate VIII, fig. C,) as they have grown 

 thicker in various parts by an accumulation of 

 blood. And this greenish mass represents that 

 which begins to give rise to the alimentary canal, 

 This is a longitudinal section of the germ, (Plate 

 VIH, fig. E ) seen from above, where we may 

 plainly observe the upper layer. We observe that 

 it is only elongated, and there is a depression fig- 

 ured here (Plate VIII, fig. A.) and below, there is a 

 collection of cells, forming what will give rise to 

 the development of the back bone. 



[PLATE VIII LAYERS AND LONGITUDINAL SEC- 

 TIONS OF THE EGGS OF BIRDS 1 



of the tube into three large cells which ccmrnuni 

 cate widely with each other (Fig. H), and will rep- 

 resent the three principal parts of the brain. 



The sides of this tube are seen first to give rise 

 backwards to some consolidation in the mass , be- 

 ing not yet organized, but being cells of a peculiar 

 form (Fig G). The number of tbese is gradually 

 increased, and in a profile view (Fig. H), you may 

 see how the cavity (Fig I) represents the cavity of 

 the spinal marrow, and how the head is bent down- 

 wards, and the tail is also gradually bent down- 

 wards, so that the germ is raised above the yolk, 

 and no longer rests flat upon the yolk, as it did in 

 the beginning. In the earlier periods, the germ 

 rests flat upon the yolk, and at first (Plate VIIL figs. 

 E) the embryo lies transversely across the egg the 

 head and the tail bend towards each other down- 

 wards, the right hand side toward the pointed end 

 of the egg, and the left hand side toward the other, 

 the blunt end. 



The successive modifications of the system thus 

 sketched out, will give rise to the formation of these 

 various systems of organs which characterize the 

 mass of the body. The parts of animal life will 

 be developed in the upper primitive uniform sub- 

 stance ; solid parts will be deposited around in 

 other places, and so the difference between hard 

 bones and contractile substance will be introduced,, 



It is, perhap?, proper for me to show how this is 

 brought about, by pointing to some figures of the 

 modifications of cells, as they occur in the animal 

 body, 



[PLATE V VARIOUS FORMS ov CELLS.] 



But the two sides of this depression growing 

 successively thicker and higher, will form, the 

 walls of a longitudinal furrow, and this furrow will 

 finally be shut up by the growth of the sides, and 

 then we have the cerebral tube gradually develop- 

 ed (Plate VII, fig. H). And you see that the head 

 is scarcely indicated (Plate VII, fig. G) by a some- 

 what greater dilatation of that upper cavity. And 

 after a while it is subdivided by the contractions 



In the beginning, the embryo consists of 

 uniform cells ; but they may be elongated ; they 

 may undergo other forms by branching; they 

 may be so elongated as to give rise to tubes ; there 

 may be various substances deposited within them, 

 and in that way they may constitute the various 

 modified different parts which compose the animal 

 body.. There are even some of these cells, which, 



