170 DEVELOPMENT AND REDUCTION OF THE TAIL 



Embryos No. 721 and No. 422, 9 mm. Crown-Rump Length. 



As no embryos of this size in sagittal section were available, I have had to 

 make use of two that were cut in coronal sections, although in them the study of 

 the vertebral structures was much more difficult. In embryo No. 721 the caudal portion is 

 cut transversely, and in the sections through the end of the tail the caudal gut with its 

 round lumen can be distinctly recognized. The extreme end of the gut is surrounded by 

 a network of capillaries communicating with the middle sacral artery and vein. The cells 

 of the caudal gut seem to fuse with those of the spinal cord and the chorda dorsalis in the 

 caudal region. In this specimen there are 37 somites and a long remnant. I was able 

 to count 32 spinal ganglia, the thirty-second being small and without nerves. In embryo 

 No. 422 there are 38 somites with a remnant, and 32 spinal ganglia. The last one of these 

 also is small and contains no nerve-fibers. The specimen is so poorly preserved that the 

 caudal gut can not be clearly made out. 



Embryo No. 1197, 10 mm. Crown-Rump Length. 



In embryo No. 1 197 there are 35 primitive vertebra- of Remak, or scleromeres, although 

 the thirty-third and thirty-fourth appear to consist each of two parts fused together. This 

 fusion has occurred, apparently, at an earlier stage. At the caudal end of the vertebral 

 column the vertel)np have not completely developed, although the tissue is condensed, 

 showing that development is well under way. The chorda dorsalis is situated dorsal to 

 the primitive vertebra- in the caudal portion and is slightly contorted. Caudally it ter- 

 minates suddenly with a rounded end ventral to the neural tube at a level with the thirty- 

 fifth vertebra. There are 32 spinal ganglia with nerves, the last 2 nerves being quite 

 delicate. I have carefully examined each section of the caudal region in an effort to locate 

 a remnant of the caudal gut, but could find no trace of it. 



Embryo No. 544, 11 mm. Crown-Rump Length. 



In embryo No. 544 the caudal end is bent sharply to the right. In the graphic recon- 

 struction shown in figiu'e 34 it is represented as straightened out in order to show more 

 clearly the relations of the structures in this region. It is pictured in a more diagranmiatic 

 way in figin-e 6. Here 38 primitive vertebrae and a remnant are present, the latter differ- 

 ing from that described in the younger embryos. In those instances I have referred to 

 it as the mesodermic remnant, using the terminology of Keibel, whereas in this stage of 

 development (9, It), and 11 nun.) the mesodermic remnant has been gradually converted 

 into a non-vertebrated tail. The last scleromere or primitive vertebra, as shown in figure 

 34, is larger than the two or three more cranially situated ones. Two theories as to its 

 development present themselves for consideration: Its growth may be the result of (1) 

 fusion of the last two or three scleromeres which have become separated from the adjacent 

 somites; (2) the addition of cells from the mesodermic remnant. WTiile somewhat in doubt 

 as to which theory to accept, I am inclined to favor the latter, as this embryo contains 38 

 primitive vertebrae, corresponding to the maximimi number of somites found in the younger 

 embryos, and there is no condensed tissue or group of cells in the end of the tail to indicate 

 the primordium of a primitive vertebra. 



Embryo No. 852, 12 mm. Crown-Rump Length. 



In embryo No. 852 I found 37 scleromeres and a remnant; also 33 spinal ganglia, the 

 last 2 being small, with slender nerves. The last 3 nerves emerge at the same point to 

 form the caudal nerves, which run from about the thirty-second to the thirty-sixth sclero- 

 mere. The central canal of the spinal cord narrows between the thirty-fourth and thirty- 

 fifth scleromeres and on the ventral wall of this narrow part are a few folds. These are 

 indicated in figure 35. The chorda is almost entirely embedded in the tissue of the primi- 



