[991 vertebral column near the dorsal surface of the last vertebral 

 body and passes thence dorsally to the ventral side of the 

 medullary cord, accompanying this nearly to the tip. In 

 contrast to the vertebral portion, the terminal portion is 

 scarcely differentiated and not well defined in the surround- 

 ing mesenchyme. 



The continuation of the aorta (ao.), i. e. the a. sacralis 

 media, at first ventral to the vertebrae, passes out into the 

 caudal filament as an a. caudalis. From this are given off 

 the segmental arteries, one for each vertebra down to and 

 including the last or thirty-sixth. (The last two are not 

 shown in the figure.) These pass up on each side of the 

 vertebral bodies, but it is doubtful if the more distal ones are 

 as yet fully open. In the same way the vena cava continues 

 into the tail, as the v. sacralis media and the v. caudalis, which 

 lies ventral and to the right of the artery. At their termina- 

 tion in the caudal filament the artery and the vein meet. The 

 vein is of 'large calibre to the region of the thirty-second 

 vertebra; here it narrows down very suddenly. There are 

 numerous small blood-vessels throughout the mesenchyme 

 of the tail. 



Emlryo JfS. Greatest Length 16 mm.; Neck-Breech Length 

 IJf. mm. — The relations of the tail to the trunk are about the 

 same as in the younger embryo first described, i. e. it is free 

 from the thirty-third vertebra on. 



The vertebral portion of the tail is longer, but the caudal 

 Lioo] filament is shorter and more shrunken. It bends sharply on 

 itself to the dorsal side, almost through an angle of 180°. 



Thirty-seven vertebrae are present, with possible indications 

 of a thirty-eighth; eight of these belong beyond doubt to the 

 coccygeal region.. The thirty-fourth and thirty-fifth are 

 partly fused in the middle. The hypapophyses of each are 

 distinct. 



The spinal ganglia number thirty-two. The relations of 

 the notochord, medullary cord and blood-vessels are the same 

 as in the embryo first described. There is a slight irregularity 

 in the notochord in the form of a process which extends 

 ventrally into the substance of the thirty-sixth vertebra. 



(12) 



