EMBRYONIC DISC 



^^^^^^P-^^^^^. 



Fig. 145. Early development of the pig. (A-E) Fate of the first two blastomeres. 

 The larger blastomere of the two-cell stage gives rise to trophoblast tissue, whereas from 

 the smaller blastomere, formative cells and trophoblast cells arise. (After Heuser and 

 Streeter, Carnegie Inst., Washington, Contrib. Embryol., 20:3.) (F) Section of two- 

 cell stage. Specimen secured from oviduct of sow, killed two days, 3'/2 hours after ovula- 

 tion. (G) Section of four-cell stage. Age is approximately IVi days. (H) Sixteen-cell 

 stage, drawn from unsectioned specimen, probably 3'/2 days old. (I) Blastular stage. 

 Specimen secured from sow, 4% days after copulation. (J-L) Stages showing the for- 

 mation of the blastocoel. (J) About 4^/4 days after copulation. (K) Six days, 1% 

 hours after copulation. (L) Six days, 20 hours after copulation. (M) Beginning dis- 

 integration of trophoblast cells over the inner cell mass and separation of entoderm cells 

 from the inner cell mass. (N) Trophoblast cells over inner cell mass almost absent, 

 entoderm forms a definite layer below inner cell mass. (O) Trophoblast cells almost 

 absent over the embryonic disc; entoderm layer continuous. (P-R) Stages shown in 

 (M), (N), (O) respectively, showing the whole blastocyst. In (Q) the entoderm cells 

 are shown migrating outward to line the cavity of the blastocyst. 



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