642 DEVELOPMENT OF THE EMBRYO. 



lopment or complete absence of the spinal cord at the same spot, 

 when it is accompanied of course by paralysis of the lower ex- 

 tremities, and almost necessarily results in early death. 



The entire skeleton is at first cartilaginous. The first points of 

 ossification show themselves about the beginning of the second 

 month, almost simultaneously in the clavicle and the upper and 

 lower jaw. Then come in the following order, the long bones of 

 the extremities, the bodies and processes of the vertebrae, the bones 

 of the head, the ribs, pelvis, scapula, metacarpus and metatarsus, 

 and the phalanges of the fingers and toes. The bones of the carpus, 

 however, are all cartilaginous at birth, and do not begin to ossify 

 until a year afterward. The calcaneum and astragalus begin to 

 ossify, according to Cruveilhier, during the latter periods of foetal 

 life, but the remainder of the tarsus is cartilaginous at birth. The 

 lower extremity of the femur begins to ossify, according to the 

 same author, during the latter half of the ninth month. The pisiform 

 bone of the carpus is said to commence its ossification later than 

 any other bone in the skeleton, viz., at from twelve to fifteen years 

 after birth. Nearly all the bones ossify from several distinct points ; 

 the ossification spreading as the cartilage itself increases in size, 

 and the various bony pieces, thus produced, uniting with each other 

 at a later period, usually some time after birth. 



The limbs appear by a kind of budding process, as offshoots of 

 the external layer of the blastodermic membrane. They are at 

 first mere rounded elevations, without any separation between the 

 fingers and toes, or any distinction between the different articula- 

 tions. Subsequently the free extremity of each limb becomes di- 

 vided 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 posterior extremities, in the human subject, are less rapid in 

 their development than the anterior. Throughout the term of 

 foetal life, indeed, the anterior parts of the body are generally more 

 voluminous than the posterior. The younger the embryo, the larger 

 are the head and upper extremities in proportion to the rest of the 

 body. The lower limbs and the pelvis, more particularly, are very 

 slightly developed in the early periods of growth, as compared with 

 the spinal column, to which they are attached. The inferior ex- 

 tremity of the spinal column, formed by the sacrum and coccyx, 

 projects at this time considerably beyond the pelvis, forming a tail, 

 like that of the lower animals, which is curled forward toward the 



