120 JOHN LEWIS BREMER 



such influence is brought to bear on the angioblast cords, and 

 here they enlarge and become the vitelline veins. 



Another agency at work in the further development of these 

 blood-vessels is shown in fig. 4. The shape of the layer of ento- 

 derm indicates a longitudinal folding of this layer to form the 

 pharynx, of which a point near l a' is to be the lateral edge, and a 

 union of the layer near c b' with a similar point on the opposite 

 side of the embryo is to complete the floor. This fold ends ante- 

 riorly by curving to the median line under the network of the first 

 aortic arch. Thus the pharynx is due, in the rabbit at least, not 

 so much to the usually described pouching forward of the ento- 

 derm as to a lateral folding of the layer, and the floor of the phar- 

 ynx is completed by a union of the entoderm of the two sides, 

 which soon fuses and forms two continuous sheets, one for the 

 floor of the pharynx, the other for the upper wall of the archen- 

 teron. 



This folding in a more advanced stage is shown by the shape 

 of the blood-vessels in figs. 5 and 6, since the vessels always lie 

 close to the entoderm. The dorsal aortae, still showing, by their 

 frequent subdivision, signs of their origin from a network of 

 vessels, are in the same relative position as before, dorsal to the 

 entoderm of the pharynx. The lateral part of the network has 

 been rolled in underneath the pharynx, whose crescentic outline 

 is marked by the plexus of vessels which forms the first arch. By 

 this folding or rolling in process the lateral edge of the network 

 now lies beneath the pharynx and near the median line, and as the 

 mesoderm makes its way between the floor of the pharynx and the 

 roof of the archenteron, new shoots from these vessels pass toward 

 the median line, and may even anastomose with others from the 

 opposite side. This ventral plexus of vessels, many of which are 

 at first solid cords, is the first indication of the ventral aorta. 

 It is connected with the lateral heart, as can best be seen in 

 fig. 6, by means of a slender vessel, the conus arteriosus, or bulb; 

 and the lateral heart has, so to speak, lagged behind in the fold- 

 ing, so that the curve of the blood-\ r essels and of the entoderm in 

 trarsverse sections of the embryo makes an S, the upper curve 

 of which comprises the dorsal aorta, first arch, and ventral aortic 



