22 Development of the Limbs, Body-wall and Back in Man 



neural processes is shown in Fig. B at the right. That portion of each 

 scleromere representing an intervertebral disc is likewise united ven- 

 trally and dorsally with its neighbors by a dense sheet of tissue at the 

 periphery of the disc. The ventral portions of these sheets of tissue 

 are represented in Fig. B. Within the space lying between the sclero- 

 meres is developed the chondrogenous tissue which gives rise to the 

 vertebral bodies. Marked alterations have taken place in the myotomes. 

 In the thoracic region the two layers of the myotome have, with the 

 exception of the dorsal and ventral tips in the more distal segments, 

 become converted into musculature (see Fig. 23 and Fig. B). As shown 

 at the left in Fig. 23, the myogenous tissue which has arisen from the 

 myotomes is being divided into three portions — a dorsal, a lateral and a 

 ventral — by the ingrowth of a vascular mesenchyme. The finer changes 

 taking place during this period are essentially similar to those previously 

 described as taking place in the body-wall of the pig.'° 



The tissue of the superficial lateral layers of the myotomes has formed 

 into a continuous layer (see Fig. C, Plate III), Segmentation, however, 

 is still visible. 



Both the costal processes of the scleromeres and the myotomes have 

 extended well into the body-wall (Fig. 23). 



The thoracic spinal nerves likewise have kept pace in growth with the 

 myotomes. The ventral extremities of the spinal nerves are caught 

 between the tips of the myotomes (see Fig. B, Plate II). From the 

 spinal nerves, dorsal and lateral branches have arisen as well as the 

 sympathetic. A sympathetic cord has arisen from the extremities of 

 the sympathetic rami. 



The segmental arteries (see Fig. 23) are similar in nature to those of 

 the stage shown in Fig. 21, but the branching is more extensive. The 

 mesenchyme is much more developed and now surrounds all formed 

 structures in each spinal segment. It has begun to invade the myotomes. 



In the region of the posterior limb bundles of fibres from the five 

 lumbar and first two sacral nerves have become anastomosed into a plex- 

 us, from which in turn four nerves have sprung. These represent the fe- 

 moral, obturator, tibial and peroneal nerves (Fig. B). Within the leg- 

 bud the central mesenchyme, near the axis of the embryo, has become 

 condensed. This condensed mesenchyme represents the femur and hip 

 bone of the adult limb. In the drawing the outline of this sclerogenous 

 mass is made dia grammatically sharp. The femoral portion of the 



'"Bardeen, Development of the musculature of the body wall in the pig, including 

 its histogenesis, and its relations to the myotomes and to the skeletal, and nervous 

 apparatus. Vol. IX, Johns Hopkins Hospital Reports, 1900. 



