416 A Three Weeks' Human Embryo 



leg-bud. The base of the hitter lies opposite the 21st to the 25th or 26th 

 myotomes. If two myotomes be considered occipital mj^otomes, the leg, 

 in this instance, lies two segments nearer the head than usual. It is 

 therefore probable that this embryo has an unusually short body-wall." 



Twenty-nine myotomes were counted and modelled on the left side, 

 and twenty-eight on the right, the discrepancy occun-ing in the caudal 

 region. This count in general, agrees with Bardeen and Lewis; 2 occi- 

 pital, 8 cervical, 10 thoracic, 5 lumbar, and 3 or 4 instead of 2 sacral, 

 as noted by them. 



The arm-pads have even a longer cephalo-caudal enlargement than 

 noted by Bardeen and Lewis, and cover the 7th to the 13th myotomes, 

 thus leaving only 7 complete myotomes between the arm- and leg-buds. 



Many of the myotomes do not model in the regular forms usually 

 shown. The first occipitals are small and imperfect (Figs. 14, 15). The 

 3d, 4th, and 5th are dorsally composed of two distinct, hollow horns 

 (Fig. 14), which merge ventrally into a common cavity. More ventrally 

 they become solid and are marked across the middle by a band of cells 

 (Fig. 15, at left). The lumen is not large until the 11th (Fig. 11), 

 from which point until near the end of the series it is a marked feature. 

 The largest and most regular myotomes are opposite the legs, the most 

 irregular among the cervical. 



In several myotomes (Figs. 5, 11), careful examination could detect 

 no limitations between a certain area at their ventral end and the con- 

 densed mesoderm of the limb-bud, in fact in these cases, the appearance 

 would indicate the origin of limb tissue from myotomes. 



Evidences of segmentation in the mesoderm are also seen cephalad of 

 the two clearly recognizable, occipital myotomes 1 and 2, and immediately 

 in line with them. Cephalad of the 1st is a minute area with apparently 

 the identical structure of a myotome, including the familiar corrugation 

 of the epidermis (Fig. 14). Still more cephalad are two other lesser 

 condensations and corresponding epidermal corrugations (Fig. 15). That 

 is, there are indications of three more occipital myotomes than are dis- 

 tinctly figured. 



Sclerotomes. — On their mesal aspect the mesodermic tissue is shrunken 

 away from the myotomes in loops (Fig. 15), and the tissue shows a con- 

 densation (sclerotome) corresponding with each loop down to the 18th 

 myotome. In the cephalic region a slight condensation occurs ventrad 

 of the notochord. That is, these last two points indicate a slight differ- 

 entiation and segmentation of sclerotogenous tissue. In some sections, 

 the continutiv of these sclerotomes with the myotomes can be seen. 



