916 GENERATION. 



existing cartilaginous structure. The development of the face will be described separately. 

 At the time when the vertebree are being developed, with their lamina? and their spinous 

 and transverse processes, the ribs extend over the thorax, and the clavicle, scapula, and 

 sternum make their appearance. 



At about the beginning of the second month, four papillary prominences, which are 

 the first traces of the arms and legs, appear on the body of the embryon. These progres- 

 sively increase in length, the arms appearing near the middle of the embryon, and the 

 legs, at the lower portion. Each extremity is divided into three portions, the arm, fore- 

 arm, and hand, for the upper extremities, and the thigh, leg, and foot, for the lower ex- 

 tremities. At the end of each extremity, there are, finally, divisions into the fingers and 

 toes, with the various cartilages and bones of all of these parts, and their articulations. 

 (See Plates I. and II., Figs. I) and II, facing page 920.) 



Very early in intra-uterine life, the skeleton, which is at first entirely cartilaginous, 

 begins to ossify, from little bony points which appear in the cartilaginous structure. The 

 first points appear at nearly the same time (about the beginning of the second month) in 

 the clavicle and the upper and the lower jaw. Similar ossific points, which gradually 

 extend, are also seen in other parts, the head, ribs, pelvis, scapula, metacarpus, and meta- 

 tarsus, and the phalanges of the fingers and toes. At birth, the carpus is entirely cartila- 

 ginous, and it does not begin to ossify until the second year. The same is true of the 

 tarsus, except the calcaneum and astragalus, which ossify just before birth. The pisiform 

 bone of the carpus is the last to take on osseous transformation, this occurring at from 

 the twelfth to the fifteenth year. As ossification progresses, the deposits in the various 

 ossific points gradually extend until they reach the joints, which remain incrusted with 

 the permanent articular cartilage. 



While the skeleton is being thus developed, the muscles are formed from the outer 

 layer of the intermediate blastodermic membrane, and the visceral plates close over the 

 thorax and abdomen in front, leaving an opening for the umbilical cord. The various 

 tissues of the external parts, particularly the muscles, begin to be distinct at the end of 

 the second month. The deep layers of the dorsal muscles are the first to be distin- 

 guished ; then, successively, the long muscles of the neck, the anterior straight muscles 

 of the head, the straight and transverse muscles of the abdomen, the muscles of the ex- 

 tremities, the superficial muscles of the back, the oblique muscles of the abdomen, and 

 the muscles of the face. 



The skin appears at about the beginning of the second month, when it is very delicate 

 and transparent. At the end of the second month, the epidermis may be distinguished. 

 The sebaceous follicles are developed at the third month ; and, at about the fifth month, 

 the surface is covered with their secretion mixed with desquamated epithelium. This 

 cheesy substance constitutes the vernix caseosa. At the third month, the nails make 

 their appearance, and the hairs begin to grow at about the fifth month. The sudoripa- 

 rous glands first appear at about the fifth month, by the formation of flask-like processes 

 of the true skin, which are gradually elongated and convoluted, until they are fully 

 developed only a short time before birth. 



Development of the Nervous System. 



We have seen, in studying the development of the spinal column, how the dorsal, or 

 medullary plates close over the groove for the neural canal. In the interior of this 

 canal, the cerebro-spinal axis is developed, by cells, which gradually encroach upon its 

 caliber, until we have remaining only the small central canal of the spinal cord, commu- 

 nicating with the ventricles of the brain. As the nervous tissue is developed in the inte- 

 rior of the neural canal, there is a separation of the histological elements at the surface, 

 to form the membranes. The dura mater and the pia mater are formed first, appearing 

 at about the end of the second month, while the arachnoid is not distinct until the fifth 



