382 H. H. Newman and J. T. Patterson. 



The belly-stalk now shows a tendency to form into" two bands 

 at the proximal or attached end. Each band later carries an umbil- 

 ical artery and vein from the placental disc to the embryo, that is, 

 they form the attachment of the umbilical cord to the wall of 

 the vesicle. The anterior margins of the bands are turned up to 

 form scroll-like structures beyond which the scale-like villi of 

 the Trager are beginning to extend out over the yolk-sac (fig. 



15 8. V.). 



There is yet to be considered the yolk sac circulation. This 

 consists of a net work of anastomosing mesodermal cords, which 

 in section are seen to be composed of a central mass of incipient 

 blood cells, surrounded on the upper side by an attenuated layer 

 of mesoderm and on the lower by the entoderm (fig. 8, b. c.). 

 These cords do not become hollowed out even at a much later 

 period than this. Indeed it is doubtful whether they ever become 

 functional blood vessels. 



In considering the details of structure we shall confine our ac- 

 counts to a brief description of a series of transverse sections of 

 the five somite embryo, and to the median longitudinal section 

 of a seven somite embryo. 



In the region of the neural fold the neural groove has become 

 greatly deepened to form the first rudiment of the brain vesicle 

 (fig. 26, n. g.), and the lateral margins of the medullary plate 

 have become tucked in beneath, thus forming a bay on each side 

 that is at once recognized as the lateral extensions of the head- 

 fold ( h. /.). In consequence of this folding the extreme lateral 

 portions of the amniotic cavity have had the marginal parts of 

 the medullary plate withdrawn from them, with the result that 

 the walls of the amnion have more or less collapsed, obliterating 

 the cavity. In all probability the obliteration is an artifact, due 

 to the rapture of the amniotic canals and the consequent escape 

 of the amniotic fluid. 



In the central region the entoderm has undergone a transfor- 

 mation to produce the notochord ( n. ch.) which consists of a row 

 of columnar cells. Already the entoderm shows signs of beginning 

 to grow beneath the notochord, so that this structure will soon 

 be cut off from the archenteron. The primordia of the pharyn- 



