818 GENERATION. 



extremities, and the thigh, leg and foot, for the lower extremities. 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 articula- 

 tions. 



Very early in intrauterine life the skeleton 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 metatarsus, and the phalanges of the fingers and 

 toes. At birth the carpus is entirely cartilaginous, 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 occur- 

 ring between the twelfth and the fifteenth years. 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 thus developing, the muscles are formed from the 

 outer layer of the mesoblast, and the visceral plates close over the thorax 

 and abdomen in front, leaving an opening for the umbilical cord. The vari- 

 ous 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 distinguished ; then successively, the long muscles of the neck, 

 the anterior straight muscles of the head, the straight and transverse mus- 

 cles of the abdomen, the muscles of the extremities, 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 epi- 

 dermis 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 con- 

 stitutes the vernix caseosa. At the third month the nails make their appear- 

 ance, 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. 



It has been 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 there remains only the small, 

 central canal of the spinal cord, communicating with the ventricles of the 

 brain. As the nervous tissue is developed in the interior 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 



