HEAD- AND TAIL-FOLDS HEART. 869 



connective-tissue, but he assumes that the material from which the latter is formed 

 is continuous protoplasm, and of equal value with the elements of the blastoderm. 



The hypoblast does not undergo any chaoge at this time ; it applies itself to 

 the inner layer of the mesoblast, as a single layer of cells, to form the splanchno- 

 pleure. 



442. FORMATION OF EMBRYO, HEART, PRIMITIVE CIRCULATION. 



Head- and Tail-Folds. Up to this time the embryo lies with its three layers 

 in the plane of the layers themselves. The cephalic end of the future embryo is 

 first raised above the level of this plane (fig. 665, V). In front of, and under the 

 head, there is an inflection or tucking-in of the layers, which is spoken of as the 

 head-fold (V, r). [It gradually travels backwards, so that the embryo is raised 

 above the level of its surroundings.] The raised cephalic end is hollow, and it 

 communicates with the space in the interior of the umbilical vesicle. The cavity 

 in the head is spoken of as the head-gut or fore-gut (V, D). The formation of 

 the fore-gut, by the elevation of the head from the plane of the three layers, occurs 

 on the second day in the chick, and in the dog on the 22nd day. The tail-fold is 

 formed in precisely the same way, in the chick on the 3rd day, and in the dog 

 on the 22nd day. The tail-fold, S, also is hollow, and the space within it is 

 the hind-gut, d. Thus, the body of the embryo is supported or rests on a hollow 

 stalk, which at first is wide, and communicates with the cavity of the umbilical 

 vesicle. This duct or communication is called the omphalo-mesenteric duct, or 

 the vitello-intestinal or vitelline duct. The saccular vesicle attached to it in 

 mammals is called the umbilical vesicle (VII, N), while the analogous much 

 larger sac in birds, which contains the yellow nutritive yelk, is called the yelk-sac. 

 The omphalo-mesenteric or vitelline duct in course of time becomes narrower, and 

 is ultimately obliterated in the chick on the 5th day. The point where it is con- 

 tinuous with the abdominal wall is the abdominal umbilicus, and where it is in- 

 serted into the primitive intestine, the intestinal umbilicus. 



[Sometimes part of the vitelline duct remains attached to the intestine, and may prove 

 dangerous by becoming so displaced as to constrict a loop of intestine, and thus cause strangu- 

 lation of the gut.] 



Heart. Before this process of constriction is complete, some cells are mapped 

 off from that part of the splanchnopleure which lies immediately under the head- 

 gut ; this indicates the position of the heart, which appears in the chick at the end 

 of the first day, as a small, bright red, rhythmically contracting point, the punctum 

 saliens, or the o-Tty/xr; klvov/mvtj of Aristotle. In mammals it appears much later. 



The heart, VI, begins first as a mass of cells, some of which in the centre dis- 

 appear to form a central cavity, so that the whole looks like a pale hollow bud 

 (originally a pair) of the splanchnopleure. The central cavity soon dilates; it grows, 

 and becomes suspended in the ccelom by a duplicature like a mesentery (meso- 

 cardium), while the space which it occupies is spoken of as the fovea cardica. The 

 heart now assumes an elongated tubular form, with its aortic portion directed 

 forwards, and its venous end backward; it then undergoes a slight /-shaped curve 

 (fig. 675, 1). From the middle of the 2nd day, the heart begins to beat in the 

 chick, at the rate of about 40 beats per minute. [It is very important to note 

 that at first, although the heart beats rhythmically, it does not contain any nerve- 

 cells.] 



From the anterior end of the heart, there proceeds from the bulbus aortae, the 

 aorta which passes forward and divides into two primitive aortae, which then 

 curve and pass backwards under the cerebral vesicles, and run in front of the proto- 

 vertebrse. Opposite the omphalo-mesenteric duct, each primitive aorta in the 

 chick sends off one, in mammals several (dog 4 to 5), omphalo-mesenteric arteries 

 (VI, A, 0), which spread out to form a vascular network within the mesoblast of the 



