146 



A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



the chorda from the medullary tube dorsally and the digestive tube ventrally can 

 be noted by a comparison of the specimens. Its dorsal separation seems to begin 

 anteriorly and to proceed caudalward. Ventrally separation apparently begins 

 in the middle region of the body and progresses both anteriorly and posteriorly. 

 Another point of ventral separation begins anteriorly and progresses caudally, 

 the last portion to become separated undoubtedly being that which is fused to 

 the pharynx. 



v.a.3 



v.a.2 



v.a.1 



&m 



ch.d 



a ~ Grf " o , * v 



!t ^: 



:-:^- > 



sS 



FIG. 0. Portion of section 42. X125 diameters. 



v.a.2 



FIG. 7. Portion of section 46. X125 diameters. 





ch.d. 



n.t 



ao.d. 



cap. 

 FIG. 8. Portion of section 218. X125 diameters. 



cl.mem. 

 FIG. 9. Portion of section 222. X125 diameters. 



Watt describes what he considers to be neurenteric canals in his twin embryos. 

 They are situated just dorsal to the cloaca and consist of a connecting rod of cells 

 joining the chorda with the medullary tube dorsally and with the entoderm 

 ventrally. This region in my specimen is cut through sagittally, but as seen in 

 text-figures 8 and 9, no evidence of such a cord of cells is present. 



