4 Development of the Limbs, Body-wall aud Back in Man 



tube the pharynx extends forwards. Trom it project the hrst and 

 second branchial pockets, Seessel's pocket, and the thyroid diverticulum. 

 Into the caudal end of the embryo the hind-gut extends. The umbilical 

 vesicle projects forwards from the region opposite the 1-6 cervical 

 myotomes (see Fig. 1). At this period the amnion arises on each side 

 along the length of the axis of the embryo as far forward as the region 

 of the heart (Fig. 1). Externally and internally the amnion is covered 

 by a layer of epithelial cells. From epithelium lining the coelom, sev- 

 eral layers of cells have arisen (Fig. 16). There is, however, as yet, no 

 true body-wall caudal to the region of the heart. There are no external 

 visible signs of limb buds. 



Embryo Lr. 



In Fig. 2 is represented the His embryo Lr; length, neck-breach, 

 4.2 mm.; age, about three weeks. The back of this embryo presents a 

 slight concavity opposite the ninth (first thoracic) myotome. It is 

 probable that this is an artifact, due to the removal of the embryo from 

 the ovum, and that in the natural condition the back curved about the 

 viscera as it does in the embryos represented in Figs. 1, 3, 4 and 5." 

 "Lr'^ shows externally thirty-one myotomes (8c, 12t, 51, 5s, Ic). The 

 ninth (first thoracic) and twenty-first (first lumbar) myotomes are desig- 

 nated by the numeral " 1.'' Lateral to the region of the myotomes 

 lies the Wolffian ridge, a band of tissue which represents the anlage of 

 the limbs and body-wall. The arm is represented by a slight swelling 

 opposite the 5th to 8th cervical and 1st thoracic myotomes. The leg is 

 represented by a slight swelling opposite the 1st to 5th lumbar and 1st 

 sacral myotomes. The amnion was probably attached, in this embryo, 

 to the umbilical cord. Between the Wolffian ridge and the umbilical 

 cord the menibrana reuniens extends, at this period, so as to cover over 

 the thoracic and abdominal viscera. It is represented as torn along 

 the heavy irregular line. 



Embryo CXLVIII. 



In Fig. 3 is represented Embryo CXLVIII; length, neck-breach, 4.3 

 mm.; age, about three weeks. A photograph of the embryo is given 

 on Plate I. Though more advanced in development than Lr, but 

 twenty-seven myotomes are present (2o, 8c, lOt, 51, 2s). This has been 

 determined by careful counting of the myotomes in serial sections of 

 the embryo. The base of the arm-bud appears to lie opposite the sev- 



4 See Mall, Human Coelom, op. cit., p. 421. 



