320 



PROFESSOR J. COSSAR EWART. 



are distinct, there is no rudiment of an allantois, and the amnion is represented by two 

 hardly perceptible lateral ridges ; as the mesoderm is not split under these ridges, 



A llantois 



Cloaca 



HORSE 



Text-fig. 17. — Diagram of the endoderm of a 21 -days horse embryo. Note the gill pouches in connection with 

 the fore-gut, the wide yolk-stalk leading into the pear-shaped yolk-sac, the cloaca, and the minute allantoic 

 diverticulum projecting from the hind-gut. 



Yr>/A -Sbc 



Yo/A-Sac 



SHEEP 



Text-fig. 18. — Diagram of the endoderm of a sheep at about the same stage of development as a 21 -days horse. 

 Note the large crescent-shaped allantois and the long tubular yolk-sac which extends into both uterine horns. 



one is perhaps hardly justified in regarding them as rudiments of the amnion. 

 Martin and Bonnet believed this 3"25 mm. embryo was 21 days old, but I have given 

 reasons for regarding it as representing the stage reached on the seventeenth or 

 eighteenth day of gestation. 



