STUDIES ON THE DEVELOPMENT OF THE HORSE. 307 



receives two branches from the caudal end of the dorsal aorta of the same side, and it 

 terminates at the caudal end of the allantoic mass in a terminal transverse sinus from 

 which the umbilical veins take their origin" (text-fig. 7). 



That the endodermic diverticulum from the hind-gut (fig. 33, al.d.) is allantoic 

 and not simply a portion of the cloacal chamber is suggested by its narrow neck, its 

 relation to the vascular and capillary plexus of blood-vessels in the allantoic meso- 

 derm, and by its agreement with the diverticulum recognised as the rudiment of 

 the allantois in the sheep and pig. 



In the chick the allantois, a small diverticulum at the close of the second day, 

 is flask-shaped and half the length of the embryo on the fifth day, while on the ninth 

 day it almost completely surrounds both embryo and yolk-sac. In the sheep and pig 

 (in which the foetal appendages appear relatively sooner than in the horse) the 

 allantois has the form of a small two-horned diverticulum on the sixteenth day and 

 of a large double-horned sac on the eighteenth day (text-fig. 15). Nevertheless, the 

 time of appearance of the allantois in the horse may be said to closely coincide with 

 that in the chick, for, though small at the stage characterised by about twenty-two 

 somites, it surrounds two-thirds of the embryo on the twenty-eighth day and nearly 

 invests the entire yolk-sac on the sixty-third day, i.e. at the stage in the horse which 

 may be said to correspond with the ninth day in the chick. 



C. The Embryo at the End of the Third Week. 



1. Size and External Conformation. 



Martin's embryo (fig. 7), which represents the phase reached in the horse at or 

 about the middle of the third week, had a total length of 3 '25 mm., and the greatest 

 breadth was I "3 mm. The 21-days embryo, instead of lying, like Martin's, at 

 right angles to the long axis of the blastocyst (fig. 4), occupied a position nearly 

 parallel to its long axis (fig. 5). Measured round the curve this embryo had a 

 length, when fresh, of 11 mm., i.e. it was more than three times the length of 

 Martin's embryo. The length of the dorsal portion of the trunk seen in fig. 10 

 was 6'25 mm. The length of the head and the adjacent part of the trunk seen in 

 fig. 11 was 3 '25 mm., and the greatest breadth 2 mm. 



If Martin's embryo represents the stage reached in the sheep at the fifteenth day 

 of gestation, the 21-days horse embryo probably represents the stage reached in the 

 sheep at the end of the eighteenth day. In the 21-days embryo horse one especially 

 notices : — (l) the curving downwards and backwards of the head and front portion of 

 the trunk round the inner ends of the vitelline veins (fig. 8) ; (2) the large pericardial 

 sac lying in the space between the branchial region and the vitelline veins (fig. 8) ; 

 (3) rudiments of the external gill slits and of the gill pouches, between the branchial 

 arches (figs. 8 and 9) ; (4) the openings into the otic sacs (fig. 11) ; (5) the absence of 

 a maxillary process from the mandibular arch ; (6) the absence of nasal sacs and of 



