340 PROFESSORS ARTHUR ROBINSON AND A. GIBSON. 



between their extremities which lies immediately adjacent to the amnion cavity is 

 part of the cloacal membrane ; in that case the entodermal allantoic diverticulum has 

 not yet formed, and the allantois is merely represented by the vascular mesoderm 

 which covers the posterior part of the cloacal chamber. Which of the two views is 

 the more correct can only be decided by the examination of somewhat older specimens, 

 and they are not at present available. In the meantime the indications are that the 

 caudal diverticulum is allantoic on account of its narrow communication with the 

 remainder of the hind-gut and its relation to a capillary plexus of blood-vessels 

 which communicate on the one hand with the dorsal aortse and on the other with 

 the umbilical veins (figs. 60, 61, PI. XIX). 



The allantoic blood-vessels consist of a number of dilated capillaries which form 

 a coarse network on each side. Each lateral network receives two branches from 

 the caudal end of the dorsal aorta of the same side, and it terminates, at the caudal 

 end of the allantoic mass, in a terminal transverse sinus from which the umbilical 

 veins take their origin (figs. 59, 60, 61, PI. XIX). But, in addition to the connection 

 with both umbilical veins through the terminal sinus, each vascular network also 

 communicates directly with the umbilical vein of the same side (fig. 60, PL XIX). 



The Notochord. 



The notochord extends from the dorsal end of the bucco-pharyngeal membrane, 

 with which it is continuous, to the growing point at the caudal extremity of the 

 embryo, where it fuses with the mass of cells in which the spinal medulla and the 

 entoderm of the caudal end of the alimentary canal also end. 



In the greater part of its extent it is a rounded cord of cells, but the cranial 

 and caudal portions differ from the longer intermediate part, which intervenes 

 between them. 



Where it lies ventral to the adjacent parts of the hind- and mid-brain it is 

 triangular in outline, in transverse section. Nearer the cranial extremity of the 

 embryo it is flattened and forms a strap-like strand, with a dorsal and ventral edge 

 and right and left surfaces. The dorsal edge lies free, ventral to the brain, whilst 

 the ventral border is fused with the entoderm of the dorsal wall of the fore-gut. Its 

 cranial extremity is fused, as previously stated, with the dorsal border of the ento- 

 dermal layer of the bucco-pharyngeal membrane. 



The caudal portion of the notochord is also cylindrical, but it is of considerably 

 greater diameter than the more cranially situated part. Shortly before it reaches 

 its caudal end, for a distance of 110/a, in the embryo, it becomes a hollow tube (text- 

 fig. 24, p. 339, and fig. 52, PI. XVIII), and this portion is bent slightly towards the 

 left side of the embryo. Its caudal extremity fuses with the mass of cells which 

 forms the growth centre at the caudal end of the body. 



