RECONSTRUCTION MODEL OF A HORSE EMBRYO TWENTY-ONE DAYS OLD. 339 



epithelium of the gut wall, the spinal medulla, and the cells of the caudal end of the 

 notochord. In other words, the epithelium of the dorsal end of the cloacal diverti- 

 culum fuses with the longitudinal growth centre of the embryo ; but the epithelium 

 which forms the caudal boundary of the diverticulum abuts against the surface 

 ectoderm of the tail amnion fold, from which it is separated, in places, by a very 

 thin layer of cells which appear to be of mesodermal nature. This portion of the 

 epithelial wall of the cloacal chamber, therefore, takes part in the formation of the 

 very small cloacal membrane which separates the cloacal chamber from the amniotic 

 cavity (text-fig. 23, p. 335, and fig. 57, PL XIX). 



The Allantois and its Blood-vessels. 



The allantois is of small size (text-fig. 24, below, and figs. 55, 56, 57, 61, PI. XIX). 

 It consists of the allantoic diverticulum from the hind-gut and the somatic mesoderm 

 which covers it. 



The allantoic diverticulum is a narrow-necked sac so flattened dorso-ventrally that 



Amnion 

 Aorta 



Spina/ Medulla 



Amnion 



Notochord 



Umbilical 

 Vein 



. Cloaca, 



UmbJlical 

 Vein 



Allantoic- 

 Mesoderm 



Allantoic 



Entoderm 



^Allantoic 

 Entoderm 



Allantoic 

 Blood Vessels 



Text-fig. 24. — Slightly oblique section through the caudal end of the embryo immediately posterior to the 

 angle between the cloacal chamber and the allantoic diverticulum. The section shows the small cloacal 

 chamber, the flattened allantoic diverticulum, and the hollow notochord. 



its cavity is reduced to the dimensions of a narrow cleft (text-fig. 23, p. 335, and 

 fig. 57, PI. XIX). 



The diverticulum commences by a narrow neck 50/a wide, but, as it passes 

 caudally, into the somatic mesoderm beneath the caudal part of the amnion, it 

 expands till it attains, in the embryo, a width of 235/a (fig. 61, PI. XIX). The ento- 

 dermal diverticulum is surrounded laterally, ventrally, and caudally with mesoderm, 

 but dorsally it lies in relation with the ectoderm in the region of the caudal end of 

 the amnion. It is possible, therefore, that both the dorsal and the ventral diverticula 

 are portions of the common cloacal chamber, and that the portion of the embryo 



TRANS. ROY. SOC. EDIN., VOL. LI, PART II (NO. 8). 



49 



