[991 Eindryo IJ^Jf. Greatest Length IJ^ mm.; Nech-Breecli 12 mm. 

 — The tail of this embryo is marked off ventrally by a fold of 

 epithelium which extends cranially from the anus, forming a 

 shallow pit or crease between the anal prominence and the 

 tail. This fold extends to the level of the cranial end of the 

 thirty-third vertebra (Fig. 6), so that from this point on, i. e. 

 distal to the third coccygeal vertebra, the caudal end of the 

 embryo projects free from the trunk. 



The vertebral column extends throughout but half the 

 length of the tail, in which, therefore, a vertebral and non- 

 vertebral portion may be distinguished. 



The terminal portion of the tail or caudal filament is bent 

 dorsally and inclined to the left side, and becoming rapidly 

 thinner distally, ends in a slight knob-like enlargement, 

 which is scarcely shown in the figure. The most conspicuous 

 structure in the caudal filament is the medullary cord, which 

 runs to the tip and there ends in a vesicular enlargement. 

 The notochord and the terminal branches of the aorta and 

 inferior vena cava also extend out into it though not so far 

 as the medullary cord. The filament is supported by a dif- 

 fuse mesenchymatous network, more concentrated in the 

 ventral side just beneath the integument, which is perhaps 

 an indication of the remains of the post-anal gut found in 

 younger embryos. 



Counting from the atlas down, it is clear that there are in 

 all thirty-six- vertebrae present, of which the distal seven be- 

 long to the coccygeal or caudal region. In the trunk, down 

 through the sacral region, the vertebral bodies are composed 

 of embryonic cartilage, which does not stain intensely. The 

 intervertebral discs, owing to the greater concentration of 

 the cells composing them, stand out in sections as deeply 

 staining bands. Between the vertebral bodies and the discs 

 there is a zone of cells, which stains more intensely than the 

 cartilage and less so than the discs. In the well advanced 

 vertebrae of the lumbar region the intermediate zone is thin 

 and clearly forms a part of the perichondrium of the vertebral 

 cartilages. Beginning with the first coccygeal vertebra this 

 intermediate or perichondrial layer forms a thick pad, especi- 



(10) 



