FURTHER DEVELOPMENT OF THE BODY. 973 



is exceedingly rare. This arrest of development gives rise to permanent separa- 

 tion of the bodies of the vertebras into two lateral halves. 



The cutaneous plates finally grow also toward the middle line of 

 the back and insinuate themselves between the muscle-plates and the 

 epidermis; in this manner the dorsal skin is formed. In the mem- 

 branous spinal column the individual cartilaginous vertebras are formed 

 successively (in man between the sixth and seventh weeks), but these 

 do not at first possess closed vertebral arches; the latter close in man 

 during the fourth month. Each cartilaginous vertebra, however, does 

 not develop from a pair of primitive vertebrae (thus, the sixth cer- 

 vical does not develop from the sixth pair of primitive vertebras); 

 but a new articulation of the spinal column takes place, and 

 in such a manner that the lower half of the preceding and the upper 

 half of the following primitive vertebra unite to form the definitive 

 vertebra. In the process of chondrification of the vertebral bodies 

 the chorda suffers a reduction, remaining larger, however, in the inter- 

 vertebral discs. The body of the first vertebra unites with that of the 

 second to form its odontoid process; in addition, it forms the anterior 

 arch of the atlas and the transverse ligament. The chorda can be fol- 

 lowed upward through the ligamentum suspensorium dentis to the 

 posterior portion of the sphenoid bone. 



The histogenetic formation of cartilage from the indifferent formative cells 

 takes place through multiplication and enlargement of the cells that finally become 

 clear nucleated vesicles. The cement-substance probably originates from the 

 union of the cells at the periphery and their outer portion (parietal substance) 

 giving off the intercellular substance. Whether the latter possesses fine canals 

 that connect the interstices of the cartilage is asserted by some and denied by 

 others. According to the statements of some investigators, the ground-substance 

 after special treatment appears to be made up of fine fibrils. 



3. In the cervical portion, on each side, there develop four cleft- 

 like openings: the visceral clefts or branchial openings. Above the clefts 

 are recesses in the lateral wall, the visceral arches (in the chick formed 

 at the end of the third day). The clefts result from rupture of the 

 fore-gut from within (although, perhaps, this does not always take 

 place in the chick, in mammals, and in man), and they are surrounded by 

 endoblastic cells. Upon the visceral arches, above and below each 

 cleft, there pass on each side the aortic arches, of which there may be 

 as many as five (Fig. 376, IX). These formations are permanent only 

 in fish. In man, all of the clefts are obliterated except the uppermost, 

 which forms the auditory canal, the tympanum, and the Eustachian 

 tube. The four visceral arches are for the greater part later transformed 

 into other formations. 



In the middle line beneath the forebrain is a thin point where 

 (in the region of the pharyngeal membrane) an invagination with an 

 embankmentlike or craterlike border first takes place, followed by rup- 

 ture, and forming the primitive oral orifice (which still comprises the 

 mouth and the nose together). Later, a depression at the caudal ex- 

 tremity (in the situation of the cloacal membrane) ruptures into the 

 hind-gut, forming the anus. Should this fail to take place, atresia ani 

 results. The lungs, the liver, the pancreas, the cecum (in birds), and 

 the allantois (to be described later) develop from the entoblast and the 

 adjacent splanchnopleure as diverticula from the primary intestinal 

 tube. The extremities appear as short stumps upon the trunk at first 

 devoid of members. 



