THE DEVELOPMENT AND REDUCTION OF THE TAIL AND OF THE CAUDAL 



END OF THE SPINAL CORD. 



By Kanae Kunitomo. 



INTRODUCTION. 



A great deal of literature has been published from time to time dealing with 

 the question of whether the human embryo at a certain stage of its development 

 has an actual tail — that is, a structure homologous with the tail of other mammals — 

 and with the persistence of such tail in after life. In biogenetic investigation this 

 is a subject of great interest (Darwin and Haeckel). It was from the assumption 

 of the occurrence of a tail in the human embryo, which was based upon one of 

 Ecker's figures (Icones Physiologic*, 1851-59), that Darwin drew one of his 

 arguments for the descent of man from a race of tailed ancestors. Von Kolliker 

 (1884) asserted that the human embryo has a tail-like process at its caudal 

 end which was not, however, recognized by him as a "true" tail. He described 

 it as eine spitze Schwanzartige Verlangerung. Ecker (1851-59) referred to it as 

 Schwanzformige Korperende, and stated that it contained only the notochord and 

 caudal end of the spinal cord, and was converted finally into a coccygeal tubercle 

 (Steisshocker) projecting caudalward. Rosenberg (1876, 1899), who investigated 

 the subject from a morphological standpoint, opposed the theory that the caudal 

 appendage in the human embryo was homologous with the tail of other mammals, 

 as he could not discover any part of the axial skeleton in the caudal projection. 

 He believed, therefore, that the latter must be concerned more with the develop- 

 ment of the spinal cord which he found embedded in its dorsal side. Ecker (1880), 

 on the other hand, held it to be a true tail, even though it contained no vertebral 

 primordia, and from his conclusions on the subject interest and discus.sion were 

 revived. His (1880rt), who coincided in general with Ecker, publi.shed his theories 

 under the heading "Besitzt der menschUche Embryo einen Hchwanz?" in his 

 Anatomic menschhcher Embryonen. He recognized a tail-hke appendage in his 

 younger specimen (4 mm.), but did not regard it as a true tail. In older embryos, 

 in which the primitive vertebrae had developed into cartilaginous tissue, he found 

 that one or two vertebrae entered into the root of the tail. This portion he desig- 

 nated as the vertebral tail. The remainder contained only notochord and medullary 

 cord (caudal filament) and was therefore called non-vertebral tail. In none of his 

 specimens did he find more than the normal number of vertebrae, 34. He states: 

 "Die Embryonen A and B haben sonach eine achte Schwanze-anlage, die aber 

 ausserordenthch kurz ist und jedenfalls nicht iiber zwei Segmentlangen umfasst." 

 The opinions of Ecker and His may be summarized as follows : 



1. The term tail refers only to that portion of the embryo which projects beyond the 

 cloaca. 



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