AND OF THE CAUDAL END OF THE SPINAL CORD. 169 



the chorda dorsalis. Each portion of the gut contains a distinct lumen. On the ventral 

 surface of the caudal region of the embryo, where the gut first begins to disappear, is a 

 small groove (indicated by X in fig. 31), which may represent the primordium of the sub- 

 caudal epithelial plate of Keibel. 



Embryo No. 371, 6.6 mm. Crown-Rump Length. 



This embryo has 37 somites and a mesodermic remnant. The thirty-seventh somite 

 lies close to the remnant, while the others are separated by narrow spaces into which blood 

 capillaries enter. The remnant is constricted at three points, the separations, however, 

 being incomplete. The caudal end of the medullary tube extends to the end of the tail, 

 as can be seen in figures 3 and 32. The caudal gut, which was distinctly recognized in the 

 5.5 nnm. specimen, has here undergone a more marked obliteration, and over its greater 

 part is left only a strand of cells which, to judge by their staining reactions, are probably 

 degenerating. The caudal portion of the gut contains a small lumen, however, and shows 

 no change from that noted in the preceding specimen. What constitutes the cranial end 

 of the shorter portion of the caudal gut in the younger embryo is here dilated and forms 

 part of the cloacal membrane. Ventral and parallel to the gut is a blood-vessel which 

 anastomoses with the middle sacral artery by means of numerous capillaries. The chorda 

 dorsalis runs within the substance of the vertebral column until it reaches the thirtieth 

 somite, when it emerges from the vertebral tissue and continues the remainder of its course 

 in the interval between the primitive vertebrae and spinal cord, until it finally loses itself 

 in the cell-mass at the end of the tail. The plexiform middle sacral artery follows a course 

 ventral to the vertebral column and can be traced to the tip of the cord. 



Embryo No. 221, 7.5 mm. Crown-Rump Length. 



Embryo No. 221 has 38 somites and a remnant, the latter showing constrictions at 

 three points. The caudal end of the embryo resembles that of a pig, as described by 

 Keibel in his work on the human embryo. The cloaca, which is situated at a level with 

 the thirty-first somite, is well developed, but as yet there has been no perforation of the 

 membrane. Below the chorda dorsalis is a short remnant of the caudal gut containing 

 a small lumen, as can be seen in figure 4, plate 1. At their caudal extremities the chorda, 

 spinal cord, and caudal gut appear to fuse together. The central canal of the spinal cord 

 is obliterated at its sacral portion, showing a pathological condition, and appears as a solid 

 mass of nervous tissue. The groove between the tail-bud and the cloaca, which in the 

 younger specimens indicated the first point of disappearance of the caudal gut, is here 

 .situated at the level of the thirty-first somite and is destined to later develop into the sub- 

 caudal epithelial plate. In this embryo there are 31 spinal ganglia with nerves. 



Embryo No. 389, 8 mm. Crown-Rump Length. 



As can be seen in figures 5 and 33, the caudal gut in embryo No. 389 still persists as 

 a group of cells inclosing a narrow lumen. This mass seems to fuse with the caudal ends 

 of the medullary tube and the chorda. The remnant of the caudal gut is surrounded by 

 a close network of capillaries, which possibly bear some relation to its absorption. At the 

 ventral wall of the spinal cord below the thirtieth somite can be seen two or three folds 

 indicated by X in figure 33. As such folds were not found in the younger specimens, I 

 assume that they begin at about this stage of development, and from now on I shall have 

 occasion to refer to them frequently. The winding chorda emerges from the vertebral 

 column at the thirty-fourth somite and continues its course to the end of the tail in the 

 interval between the vertebral column and the spinal cord. This embryo contains 38 

 somites and a mesodermic remnant. In the latter I was unable to make out any con- 

 strictions, only a mass of germinating cells. The spinal ganglia number 32. 



