DEVELOPMENT OF THE NERVOUS SYSTEM. 671 



ossify from several distinct points ; the ossification spreading as the 

 cartilage increases in size, and the bony pieces, thus produced, uniting 

 with each other at a later period, usually some time after birth. 



The limbs appear by a budding process from corresponding parts of 

 the body. They are at first mere rounded elevations, without any 

 separation between the fingers and toes, or distinction between the 

 articulations. Subsequently the free extremity of each limb becomes 

 divided into the phalanges of the fingers or toes ; and afterward the 

 articulations of the wrist and ankle, knee and elbow, shoulder and hip, 

 appear successively from below upward. 



The lower limbs in man are less rapid in development than the 

 upper. Both the legs and the pelvis are very slightly developed in 

 the early periods of growth, as compared with the spinal column to 

 which they are attached. The inferior extremity of the spinal column, 

 formed by the sacrum and coccyx, projects at first beyond the pelvis, 

 forming a tail, which is curled forward toward the abdomen, and ter- 

 minates in a pointed extremity. The entire lower half of the body, 

 with the spinal column and appendages, 

 is also twisted, from left to right ; so that _ Fl <*- 2 

 the pelvis is not only curled forward, but 

 also faces at right angles to the direction 

 of the head and upper part of the body. 

 Subsequently the spinal column becomes 

 straighter and loses its twisted form. At 

 the same time the pelvis and its muscular 

 coverings grow so much faster than the 

 sacrum and coccyx, that the latter become 

 concealed under the adjoining soft parts, 

 and the rudimentary tail disappears. HUMA* EMBRYO, about one month 



The integument of the embryo is at first old. showing the large size of the 

 thin, vascular, and transparent. It after- head and upper parts of the body ; 



the twisted form of the spinal col- 

 becomes thicker, Whitish, and more umn ; the rudimentary upper and 



Opaque. Even at birth it is Considerably lower limbs -and the rudimentary 



J tail at the end of the spinal column. 



more vascular than in the adult condition, 



and has a strongly marked ruddy color, due to its transparency and 

 the abundance of its capillary blood-vessels. The hairs begin to 

 appear about the middle of intra-uterine life ; showing themselves 

 first on the eyebrows, afterward on the scalp, trunk, and limbs. The 

 nails are in process of formation from the third to the fifth month ; and, 

 according to Kolliker, are covered with a layer of epidermis until after 

 the latter period. The sebaceous matter of the cutaneous glandules 

 accumulates upon the skin after the sixth month, and forms a whitish, 

 semi-solid, oleaginous layer the vernix caseosa, which is most abun- 

 dant in the flexures of the joints, between the folds of the integument, 

 behind the ears, and on the scalp. 



