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PROFESSOR J. COSSAR KWART. 



t ion of the amnion in the horse until the middle of the third week. Even when four 

 pairs of somites are present the amnion in the horse is only represented by very 

 slight lateral ridges (text-fig. l), but by the twenty-first day the amnion is complete. 



Text-fig. 14. —A 14-days 22-hours sheep embryo with two mesodermic somites (ra.so.), a complete 

 amnion (a>n.), and an allantoic diverticulum (al.d.). (After Bonnet.) 



Text-FIG. 15. — A 17-days 23-hours sheep embryo, with about as many somites as a 21-days horse. 

 Note the large allautois (all.) in connection with the hind-gut [h.g.), the amnion (am.), and 

 the yolk-stalk (y.s.). (After Bonnet.) 



all 



TEXT-FIG. 16. — Diagram of a 21-days sheep embryo and its membranes. Compare with diagram of a 21-days horse embryo 

 (fig. 34). A 21-days sheep is at about the same stage of development as a 28-days horse, am,, amnion ; clt., chorion con- 

 sisting of a trophoblast (fr. ), and somatic mesoderm (so.m.) ; y.s., yolk-sac ; all., allantois. 



The early rupture of the zona pellucida and the absence of an albumen layer may 

 perhaps account for the amnion appearing at an earlier phase in the sheep than in 

 the horse. 



Yolk-sac and Mesoderm. — In the sheep the splitting of the mesoderm already 



