VISCERAL CLEFTS AND ARCHES. 8/1 



Lastly, parts of the somatopleures also grow towards the middle line of the back, 

 and insinuate themselves between the muscle-plate and the epiblast ; thus, the 

 dorsal skin is formed (Remak). 



In the membranous vertebral column, there are formed the several cartilaginous 

 vertebrae, the one behind the other, in man at the 6th to 7th week, although at 

 first they do not form closed vertebral arches ; the latter are closed in man about 

 the 4th month. Each cartilaginous vertebra, however, is not formed from a pair 

 of protovertebrae, i.e., the 6th cervical vertebra from the 6th pair of protovertebrae, 

 but there is a new subdivision of the vertebral column, so that the lower half of 

 the preceding protovertebra and the upper half of the succeeding protovertebra 

 unite to form the final vertebra. While the bodies are becoming cartilaginous the 

 chorda becomes smaller, but it still remains larger in the intervertebral discs. The 

 body of the first vertebra or atlas unites with that of the axis to form its odontoid 

 process, and in addition it forms the arcus anterior atlantis and the transverse 

 ligament (Hasse). The chorda can be followed upwards through the ligamentum 

 suspensorium dentis as far as the posterior part of the sphenoid bone. 



The histogenetic formation of cartilage from the indifferent formative cells takes place by 

 division and growth of the cells, until they ultimately form clear nucleated sacs. The cement 

 substance is probably formed by the outer parts of the cells (parietal substance) uniting and 

 secreting the intercellular substance. It is supposed by some that the latter contains fine 

 canals, which connect the protoplasm of the adjoining cells. 



Visceral Clefts and Arches. Each side of the cervical region contains four 

 slit-like openings the visceral clefts or branchial openings (Bathke); in the chick, 

 the upper three are forme'd at the 3rd, and the fourth on the 4th day. Above the 

 slits are thickenings of the lateral wall, which constitute the visceral or "branchial 

 arches (fig. 671). The clefts are formed by a perforation from the fore-gut, but 

 this, perhaps, does not always occur in the chick, mammal, and man (His), and 

 they are lined by the cells of the hypoblast. On each side in each visceral arch, 

 i.e., above and below each cleft, there runs an aortic arch, five on each side 

 (fig. 665, IX). These aortic arches persist in fishes. In man, all the slits close, 

 except the uppermost one, from which the auditory meatus, the tympanic cavity, and 

 the Eustachian tube are developed. The four visceral arches are for the most part 

 made use of later for other formations (p. 879). 



Primitive Mouth and Anus. Immediately under the fore-brain, in the middle 

 line, is a thin spot, where there is at first a small depression, and ultimately a 

 rupture, forming the primitive oral aperture, which represents both the mouth 

 and the nose. Similarly, there is a depression at the caudal end, and the depres- 

 sion ultimately deepens, thus communicating with the hind-gut to form the anus. 

 When the latter part of the process is incomplete, there is atresia ani, or imper- 

 forate anus. Several processes are given off from the primitive intestine, including 

 the hypoblast and its muscular layers, to form the lungs, the liver, the pancreas, 

 the caecum (in birds), and the allantois. 



The extremities appear at the sides of the body as short un jointed stumps or 

 projections at the 3rd cr 4th week in the human embryo. 



444. AMNION AND ALLANTOIS. Amnion. During the elevation of the 

 embryo from its surroundings, immediately in front of the head (at the end of the 

 2nd day in the chick), there rises up a fold consisting of the epiblast and the outer 

 layer of the mesoblast, which gradually extends to form a sort of hood over the 

 cephalic end of the embryo (fig. 665, VI, A). In the same way, but somewhat 

 later, a fold rises at the caudal end, and between both along the lateral borders 

 similar elevations occur, the lateral folds (fig. 665, III, A). All these folds grow 

 over the back of the embryo to meet over the middle line posteriorly, where they 

 unite at the 3rd day, in the chick, to form the amniotic sac. Thus, a cavity which 



