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PROFESSOR J. COSSAR EWART. 



(fig. 28) occupied by the unsplit highly vascular mesoderm ; beyond the sinus it is 

 in contact with the yolk-sac endoderm (fig. 34). 



In the sheep, up to the appearance of the amnion folds, the trophoblast, except 

 under the embryonic shield, is in contact with a thin imperfect layer of endoderm. 



so 



am 



Text-fig. 12. — Drawing to indicate the stage reached in the horse at the end of the third week, after the albumen coat as 

 well as the zona has disappeared, am., amnion, now complete ; ch. , chorion, made up of a layer of trophoblast and a 

 layer of somatic mesoderm ; s.c, spinal cord ; d.a., dorsal aorta; sp., splanchnopleure ; tr., trophoblast; incs., unsplit 

 vascularised mesoderm ; end., endoderm. 



SO 



TEXT-FIG. 13. — Drawing to indicate the stage reached in the sheep about the middle of the 

 third week. In the 21 -days horse the mesoderm is only split in the vicinity of the 

 embryo. In the sheep at a corresponding stage the splitting of the mesoderm is com- 

 plete ; the inner (splanchnic) layer has united with the endoderm to form a free yolk- 

 sac vesicle (y.s.), the outer with the trophoblast to form the chorion (ch. ). am., amnion ; 

 s.c, spinal cord ; d.a., dorsal aorta ; sp., splanchnopleure. (After Bonnet.) 



As the splitting of the mesoderm proceeds, the exocoelom extends in all directions, 

 with the result that the trophoblast is completely separated from the endoderm and 

 provided with a lining of non-vascular mesoderm (text-fig. 13). 



In the horse the trophoblast at the end of the second week seems to consist of 

 simple polygonal nucleated cells. At the middle of the third week the cells are 

 cubical, but by the end of the third week they are long enough to rank as columnar 

 cells. Up to the sinus terminalis there are simple columnar cells (fig. 28) ; beyond 

 the sinus, in addition to typical columnar cells, there are very tall columnar cells 



