190 



DEVELOPMENT AND REDUCTION OF THE TAIL 



initial form of the ventriculus terminalis. Tlic portion of the si)inal cord which 

 incloses the ventriculus is the i)riniitive conus meduUaris. 



The ventriculus terminalis was found by Argutinsky in a 45 nmi. embryo; by 

 Brugsch and Unger in a 25 mm. embryo, and what may be considered as its primor- 

 dium is already apparent in my specimens of 11 and 12 mm. respectively, as shown 

 in figures 34, 35, and 36. It can be seen from stage to stage retreating cranialward, 

 while the atrophic canal gradually lengthens. In the 18, 23, and 25 mm. embryos 

 the latter expands noticeably in the median or caudal region, as shown in figures 

 39, 40, and 42. This was also observed by linger and Brugsch, who, however, 

 did not regard it as the primordium of the ventriculus terminalis, but rather as a 

 homologue of the sinus terminahs of the amphibians, w^hich develops at the caudal 

 end of the central canal of the spinal cord. In the majority of my sjDecimens from 

 18 to 30 mm. the caudal end of the atrophic canal shows diverticula such as those 

 descrii^ed by Unger and Brugsch. In such embryos the spinal cord becomes 



temporarily longer than the vertebral col- 

 umn. It seems probable, therefore, that 

 in the wall of the atrophic portion of the 

 cord the ependymal cells increase rapidly 

 by proliferation, and perhaps also by the 

 migration of other ependymal cells from 

 the more caudal part of the tail, which 

 is in process of regression. By reason of 

 these two processes folds develop in this 

 wall, such as are well shown in text-figure 1 . 

 In these embr^yos the caudal end of the 

 chorda dorsahs seems to exert an attrac- 

 tion upon the caudal end of the medullary 

 tube, thus drawdng it into a more cranial 

 position (fig. 39). In embryos of 30, 33, 

 35, and 37 mm. the atrophic canal is longer 

 and narrower than m the slightly younger 



specimens, but the caudal end is still di- 



lated. At several points the canal has be- 

 come so narrowed that its central cavity 

 is obhterated and gradually becomes con- 

 verted into a cell-strand, as shown in fig- 

 ure 44. 



At the stage where the embryo has 

 entirely lost its external tail the spinal 

 cord is about the same length as the ver- 

 tebral column, as shown clearly in the 30 and 37 mm. specimens. From this time 

 on the vertebral column increases relatively in length, although there is no cessa- 

 tion of growth of the spinal cord. In the 37 mm. embryo a l^undle of nerve-fibers 

 (i. e., marginal zone) is visible on the ventral side of the atrophic cord, as shown in 



Text-fiuvre 1. 



Sagittal section through caudal end of a human embryo 

 18. mm. long (Carnegie Collection, No. 432. slide 22, 

 row 1, sec. 3), enlarged 50 diameters. The wall of 

 the atrophic portion of the spinal cord is thinner 

 and represents a much younger stage of develoi>- 

 nient than that of the main part of spinal cord. 

 From this time on it undergoes gradual regressive 

 changes with obliteration of the central canal and is 

 eventually replaced by a simple fibrous strand. 



