186 DEVELOPMENT AND REDUCTION OF THE TAIL 



berg, Ecker, Keibel, and others — that the tail and the coccygeal tubercle in human 

 embryos become shorter and finally disapj^ear by an increase in volume of the 

 caudal soft tissues, muscular tissue, subcutaneous connective-tissue, etc., which 

 surround the caudal part of the ^'ertebral column. 



At an earlier stage the swelling between the primitive anus and the root of the 

 tail is called the post-anal swelling. Keibel asserts that in embryos 11 mm. and 

 larger the root of the tail is separated from the ventral trunk by double j^lates of 

 epithelial cells which lie between it and the anus. Therefore, by means of these 

 plates, which consist of two sheets of epidermal cells connected ventrally to the 

 post-anal epidermis and dorsally to the ventral side of the tail, the tail-root is dis- 

 tinctly marked off. Following Keibel's idea, Tourneux speaks of it as depression 

 sous candale de Vintecjument externe. Unger and Brugsch describe the stages of 

 disappearance of this post-anal swelling in embryos 25 and 45 mm. long. In my 

 specimens it is quite clear. In the 13 and 14 mm. embryos these plates are visible, 

 but in the 45 mm. specimen they have disappeared. After observing the specimens 

 in the various stages, my conclusions on this point are as follows: At a certain 

 stage (13 mm. and older) the digestive tube grows more rapidly than the vertebral 

 canal, so that the depression gradually straightens out. At this time the caudal 

 region of the digestive tract — viz, the cloacal region — develops faster than the 

 caudal end of the vertebral column, which constitutes the caudal end of the 

 internal tail. Moreover, the mesodermic tissue between the primitive anus and the 

 root of the tail develops rapidly and gradually bulges downward. By the swell- 

 ing of the caudo-ventral region of the tail-root the fold of epidermis, or so-called 

 epithelial plate, is stretched by degrees and at last disappears. The growing of 

 the coccygeal tubercle would also aid in this process. In his paper Keibel asserts 

 that the mesodermic tissue between the primitive anus and the tail-root grows 

 luxuriantly at certain stages and bulges downward. He terms this swelling die 

 ■postanalen wulst (post-anal swelling). In this way the epithelial plates disappear. 

 This epidermal plate between the anus and tail-root moves gradually caudal ward. 

 In the 12 mm. embryo it is situated at the level of the thirty-third vertebra, and in 

 the 46 mm. specimen has moved down to the level of the thirty-fourth vertebra. 

 The caudal end of the rectum — viz, the caudal end of the digestive tract, and perhaps 

 that of the genito-urinary organs as well — has likewise moved caudalward. 



(3) Originally the caudal portion of the vertebral column is nearly a straight 

 line, but in embryos about 20 mm. long the axis of the column shows a distinct 

 ventral flexion at about the level of the thirtieth or thirty-first vertebra. There 

 Is a second flexion which is dorsal at the caudal end of the vertebral column, 

 between the vertebrated portion and the lost-vertebrse portion of the tail, which 

 is seen in younger embryos. The caudal remainder of the lost-vertebrse tail has, 

 therefore, moved to the dorsal side of the vertebral column, being joined to the 

 last vertebra by bands of embryonic connective-tissue. These bands are the so- 

 called caudal Ugament. In these embryos the ridge or epidermal plate between 

 the coccygeal tubercle and the rectum has become shallow almost to the point of 

 disappearance, as shown in figures 40 and 42. In the 30, 33, and 39 mm. embryos 

 the lost-vertebree portion of the tail has almost entirely disappeared from the sur- 



