182 DEVELOPMENT AND REDUCTION OF THE TAIL 



oblique course from the periphery to the center of the vertebral canal. There is thus laid 

 out the early form of the dural sac. Outside of this sac the fibers are separated into tufts 

 which run parallel and caudalward. In the space between the dural sac and the conus 

 meduUaris the arachnoid membrane can be seen developing. The pia mater envelops 

 closely the spinal cord and supports the blood-vessels; between the twenty-fifth and twenty- 

 eighth vertebrae it is separated from the dura mater and the arachnoid by a still wider space . 



Embryos No. 662, 80 mm. Crown-Rump Length; No. 928, 100 mm. Crown-Rump Length; 



No. 142, 125 mm. Crown-Rump Length. 



As the investigation with embr>'os Nos. 662, 928, and 142 was not ver>^ satisfactory, 

 I shall not attempt to give its results in detail at this time. I have, however, made a special 

 study of the coccygeal medullary vestige because of its importance in comparison with 

 the same structure in younger specimens. In the 80 and 100 mm. embryos the coccygeal 

 vestige is very small and its contained cavity narrower than in the younger specimens. 

 In the 125 nun. embryo (negro) the structure is well developed and shows one long offshoot 

 stretching under the epidermis at the sacral region. It is quite different in form and 

 condition from the case reported by Tourneux (Precis d'embrj^ologie humain), and there- 

 fore does not present the loop formed by a more deeply situated limb {segment coccygien 

 direct) and a more superficial limb (segment coccygien refleche). In this embryo the 

 coccygeal vestige contains a slender cavity. 



DEVELOPMENT AND REDUCTION OF THE TAIL. 



In considering the process of reduction of the tail I should like, in the first 

 place, to refer to the important study of this condition in mammals made by Braun 

 (1882), w'hose conclusions in general are as follows: 



(1) The tail of the mammalian embrj'o consists of two portions — a vertebrated part 

 and a non-vertebrated part, the latter situated caudal to the former. 



(2) The non-vertebrated part appears usually in the form of a thread at the end of 

 the vertebrated tail, and consequently may be designated the caudal filament {Schwanz- 

 faden). Being usually thinner than the tail itself, it is consequently sharply marked off 

 from the latter. 



(3) The vertebrated part of the tail can again be subdivided into two parts according 

 to whether it projects from the body or not. The projecting part is designated as tail, 

 although it has long been well known that this is directly continuous with the sacral verte- 

 brae. The relative size of the internal and external tail varies, and hence we meet with 

 long-tailed, short-tailed, and tailless mammals. 



(4) The caudal filament is a transitory' structure, although for a time it contains the 

 end of the spinal cord, the chorda dorsalis, and the caudal gut. These structures undergo 

 resorption and the last tissue to persist is the epidermis, the caudal thread for a time per- 

 sisting of only epidermis cells. 



(5) The caudal gut originally extends into the tail; before being resorbed it separates 

 into fragments which disappear, the last to persist being the part constricted off at the tip 

 of the tail. 



(6) The chorda dorsalis always projects beyond the caudal vertebrae, where it separates 

 into forked processes or curls in irregular loops. This part disappears completely. 



(7) The spinal cord originally extends to the tip of the tail. The latter, however, 

 soon exceeds it in length, when it terminates at the base of the caudal filament. It was 

 possible to show in sheep embryos that the ascensus meduUae is due not alone to the over- 

 growth of the vertebrae, but that also there is degeneration and absorption of the caudal 

 end of the spinal cord, to which in part the formation of the filum terminale owes its origin. 



