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PROFESSOR J. COSSAR EWART. 



and text-fig. 12) into somatic and splanchnic layers, thus giving rise to coelomic and 

 exocoelomic spaces. Owing to this partial splitting of the mesoderm the part of the 

 yolk-sac nearest the embryo is complete, i.e. it consists of a layer of endoderm and a 

 layer of splanchnic mesoderm. But beyond the exocoelom (fig. 34 and text-fig. 12) the 

 yolk-sac at the end of the third week is not yet provided. witli its mesodermic coat, for 

 up to a line slightly beyond the sinus terminalis it simply consists of a single layer 



_ ^jaffsT • • • •• « • • * 





Text-fig. 4. — Sinus terminalis (s./. ) of a 21-days embryo and trophoblastic disc (t.d.) seen from within. 

 b. v., blood-vessels on growing margin of unsplit mesoderm beyond sinus terminalis. x 90. 



of endoderm which forms the inner boundary of a space occupied by the unsplit 

 mesoderm ; beyond this line the endoderm is intimately related to the trophoblast 

 (fig. 28).* The cells of the yolk-sac endoderm around the opening into the yolk- 

 stalk, like the cells lining the stalk, are columnar in form (fig. 50) ; but within a short 

 distance of the yolk-stalk, right up to the sinus terminalis, the yolk-sac endoderm 

 consists of a single continuous layer of fiat polygonal cells (fig. 31, a) some of which 



* Iu the sheep and pig the mesoderm is soon completely split into somatic and splanchnic layers. The result 

 of this splitting is the formation of a free yolk-sac vesicle (text-fig. 13). The yolk-sac is nearly, but never quite, 

 a free vesicle in the horse. 



