172 DEVELOPMENT AND REDUCTION OP THE TAIL 



not go to make up the primitive vertebrae, and constitutes the primordium of the caudal 

 ligament. As is well known, the sclerotomes of the somites develop into not only primi- 

 tive vertebrae, but also into several other supporting tissues which form the framework of 

 the vertebral column. (Keibel and Mall, Human Embryology, I, page 331.) 



The spinal cord narrows suddenly at the thirty-second vertebra, so that the portion 

 caudal to this, together with its canal, presents an appearance distinctly different from the 

 main body of the cord, as can be seen in figure 36. On account of its regressive appear- 

 ance, I shall hereafter refer to this part as the atrophic cord. One of its very character- 

 istic features is a slender, narrow canal, and it might be desirable to speak of this as the 

 narrow canal portion of the spinal cord. 



Embryo No. 406, 16 mm. Crown-Rump Length. 



A graphic reconstruction of embryo No. 406 is shown in figure 37, and a more dia- 

 grammatic sketch in figure 9. These show that the embryo has 36 cartilaginous vertebrae. 

 On the right side the thirty-second and thirty-third segments have fused together. The 

 last vertebra consists of two or three sections, each of which in an earlier stage probably 

 represented a comjjlete somite, these later fusing into one large segment. At the caudal 

 end of this is a group of undifferentiated mesodermal cells — the primordium of the caudal 

 ligament. The caudal end of the chorda dorsahs emerges at the thirty-sixth vertebra 

 and terminates abruptly between the vertebral column and the spinal cord. The spinal 

 cord narrows suddenly at the thirty-fourth vertebra. On the ventral wall of the canal in 

 this narrow or atrophic portion of the cord there are three or four folds (fig. 37). In some 

 of the sections can be seen a larger fold at the level of the twenty-ninth vertebra, which 

 hangs down to the level of the thirty-fourth, both sides adhering to the wall of the spinal 

 cord, thus forming a diverticulum, which is not shown in the illustration. The caudal 

 end of the emtjryo is bent sharply dorsal, the bent portion being marked off on the surface 

 by a shallow, circular furrow. The spinal cord extends to the end of the tail, conforming 

 to the shape of the bent portion. The extreme end contains a narrow cavity which rep- 

 resents the caudal end of the central canal; the canal is interrupted at the root of the tail, 

 where the cord appears to consist of solid nerve-tissue, as is indicated in figure 37. In this 

 specimen there are 31 spinal ganglia with distinct nerves. 



Embryo No. 43, 16 mm. Crown-Rump Length. 



This specimen has 37 cartilaginous vertebrae, the last being divided into three parts. 

 There are 32 spinal ganglia. A graphic reconstruction was made of this embryo, but it is 

 not illustrated in the figures. 



Embryo No. 576, 17 mm. Crown-Rump Length. 



This embryo has 35 cartilaginous vertebrae, the last consisting of three small pieces 

 fused together. The demarcation between these pieces can be more clearly recognized 

 in the lateral portions of the column than in the median plane; so in determining the com- 

 position of the last segment one must study carefully the more lateral line of sections. A 

 profile reconstruction of the embryo is shown in figure 38, and a more diagrammatic sketch 

 in figure 10. The tail, with the caudal end of the spinal cord, is bent sharply dorsalward. 

 The spinal cord narrows suddenly at the thirty-second vertebra and from this point down 

 the central canal, which extends the entire length of the cord, becomes much smaller 

 and rounder, while in the more cranial portion a transverse section of it would form an 

 elongated oval. The ventral wall of the canal in the atrophic portion presents several 

 transverse folds, as seems to be usually the case at this stage of reduction. The caudal 

 portion of the chorda dorsalis is convoluted and its end sharply retracted. 



