870 



FORMATION OF THE BODY. 



umbilical vesicle. From this network, there arise the omphalo-mesenteric veins, 

 which run backwards on the vitelline duct, and end by two trunks in the venous end 

 of the tubular heart. In the chick, these veins arise from the sinus terminalis of the 

 future vena terminalis of the area vasculosa. Thus, the first or primitive circu- 

 lation is a closed system, and functionally it is concerned in carrying nutriment and 

 oxygen to the embryo. In the bird, the latter is supplied through the porous shell, 

 and the former is supplied up to the end of incubation by the yelk. In mammals, 

 both are supplied by the blood-vessels of the uterine mucous membrane to the ovum. 

 In birds, owing to the absorption of the contents of the yelk-sac, the vascular area 

 steadily diminishes, until ultimately, towards the end of the period of incubation, 

 the shrivelled yelk-sac slips into the abdominal cavity. In mammals, the circula- 

 tion on the umbilical vesicle, i.e., through the omphalo-mesenteric vessels, soon 

 diminishes, while the umbilical vesicle itself shrivels to a small appendix, and the 

 second circulation is formed to replace the omphalo-mesenteric system. The first 

 blood-vessels are formed in the chick, in the area vasculosa, outside the position 

 of the embryo, at the last quarter of the first day, before any part of the heart is 

 visible. The blood-vessels begin in vaso- formative cells [constituting the " blood- 

 islands " of Pander]. At first they are solid, but they soon become hollow ( 7, A). 



A n a rrow- meshed plexus of lymphatics is formed in the area vasculosa of the chick (His), and 

 it communicates with the amniotic cavity (A. Budge). 



443. FORMATION OF THE BODY. Body-Wall. (1) The coelom, or 

 pleuro-peritoneal cavity, becomes larger and larger, while at the same time, the 

 difference between the body- wall and the wall of the intestine becomes more pro- 

 nounced. The latter becomes more separated from the protovertebrae, as the 

 middle plate begins to be elongated to form a mesentery. The body-wall, or 

 somatopleure, composed of the epiblast and the outer layer of the cleft mesoblast, 

 becomes thickened by the ingrowth into it of the muscular layer from the muscle- 

 plate, and the position of the bones and the spinal nerves from the protovertebrae. 

 These grow between the epiblast and the outer layer of the mesoblast (Remak). 

 [The somatopleure, or parietal lamina, from each side grows forward and towards 

 the middle line, where they meet to form the body-wall, while at the same time, 

 the splanchnopleure, or visceral lamina, on each side also grow and meet in the 

 middle line, and when they do so, they enclose the intestine. Thus, there is one 

 tube within the other, and the space between is the pleuro-peritoneal cavity.] 



(2) Vertebral Column. A dorsally placed structure, called the muscle-plate 

 (fig. 667, ms.\ is differentiated from each of the protovertebrae ; the remainder of 

 the proto vertebra, the protovertebra proper, coalesces with that on the other side, 

 so that both completely surround the chorda, to form the membrana reuniens 

 inferior, in the chick on the 3rd, and in the rabbit on the 10th day, while, at the 

 same time, they close over the medullary tube dorsally, in the chick at the 4th 

 day, to form the membrana reuniens superior (Reichert). Thus, there is a union 

 of the masses of the protovertebrae in front of the medullary tube, which encloses 

 the chorda, and represents the basis of the bodies of all the vertebrae, whilst the 

 membrana reuniens superior, pushed between the muscle-plates and the epiblast on 

 the one hand and the medullary tube on the other, represents the position of the 

 entire vertebral lamince as well as the intervertebral ligaments between them. In 

 some rare cases the membrana reuniens superior is not developed, so that the 

 medullary tube is covered only by the epiblast (epidermis), either throughout its 

 entire extent, or at certain parts. This constitutes the condition of spina bifida, 

 or, when it occurs in the head, hemicephalia. The vertebral column at this 

 membranous stage is in the same condition as the vertebral column of the cyclo- 

 stomata (Petromyzon). The membranes of the spinal cord, the spinal ganglia, and 

 spinal nerves are formed from the membrana reuniens superior. 



