AND OF THE CAUDAL END OF THE SPINAL CORD. 173 



Embryo No. 991, 17 mm. Crown-Rump Length. 



The caudal end of embryo No. 991 is somewhat torn, but I feel reasonably sure that 

 it did not exhibit a long caudal process. At a level between the thirty-second and thirty- 

 third vertebrae the central canal of the spinal cord narrows suddenly, and on the ventral 

 wall of the atrophic portion of the cord are two folds. There are only 31 spinal ganglia, 

 the first cervical on each side being absent. The others are completely developed, and 

 even the thirty-first has its full complement of nerve-fibers. In embryo No. 576, just 

 described, this nerve was quite slender. The chorda dorsalis runs in a straight line through 

 the cartilaginous vertebral column and emerges from the thirty-fourth vertebra without 

 winding. This is the first specimen of the series that lacks the non-vertebrated tail. At 

 the point where the tail is found in younger embryos this has a small projection resembling 

 a tail-bud more than an actual tail. Between the last vertebra and this caudal projection 

 is a mesodermic cell-mass into which enter the plexif orm branches of the middle sacral artery 

 and vein. 



Embryo No. 432, 18 mm. Crown-Rump Length. 



Embryo No. 432 contains 34 cartilaginous vertebra?. The last is larger than the 

 thirty-third and on its lateral side shows three divisions, each consisting of young precar- 

 tilaginous tissue. The middle part, where the segments are partially fused, is made up 

 of cartilaginous tissue, and here the vertebra is incompletely divided into two segments — 

 cranial and caudal. A little to one side of the median line, therefore, it is possible to count 35, 

 and more laterally, 36 vertebrae. If the scleromeres of the several somites fuse together 

 and develop into the last cartilaginous vertebra, we may assume that the remaining 

 mesenchymal somites left in the caudal portion of the tail form the non-vertebrated 

 tail, and that in a more advanced stage of embryonic growth this substance develops into 

 the caudal ligament. The chorda dorsalis shows two branches at its caudal end; one is 

 formed at the thirty-second, the other at the thirty-fourth vertebra, and both follow a 

 dorsal course. The more caudal branch is the longer, and its pointed extremity, which 

 represents the caudal end of the chorda, adheres to the ventral wall of the atrophic por- 

 tion of the spinal cord, as shown in figure 39. The wall of the spinal cord is quite thick in 

 this region and at its upper portion are several folds. This condition is very interesting 

 on account of its possible mechanical relation to the chorda, because as the caudal end of 

 the chorda retracts it would tend to draw up the end of the spinal cord that is in the tail. 



Embryo No. 431, 19 mm. Crown-Rump Length. 



Embryo No. 431 has 33 cartilaginous vertebrae. The last is larger than the thirty- 

 second and consists of two segments. It would seem most probable, therefore, that it has 

 been developed by the fusion of two or more parts. The chorda dorsalis is considerably 

 distorted in the thirty-third vertebra, thus indicating a fusion of several primitive verte- 

 brae, and its caudal end adheres to the ventral wall of the spinal cord (fig. 13). The spinal 

 cord extends to the tip of the tail, and a short distance from its extremity the ventral wall 

 shows several folds. It appears possible that with the retraction of the chorda dorsalis 

 the neural tube is also drawn up, thus producing folds on its ventral wall. The caudal 

 end of the anterior spinal artery winds through these folds. The primordium of the ven- 

 triculus terminalis, between the wide and narrow parts of the central canal, is seen at the 

 level of the thirty-second vertebra. This embryo has 32 spinal ganglia, the thirty-second 

 being incomplete and without nerve-fibers; in all the others, however, the spinal nerves 

 are complete. 



Embryo No. 837, 21 mm. Crown-Rump Length. 



Embryo No. 837 has 35 cartilaginous vertebrae, but the last is very small and its 

 transition into cartilage is just beginning. It is surrounded by a voluminous mass of 

 precartilaginous tissue resulting from the fusion of the last few scleromeres. The caudal 



