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TEXT-BOOK OF EMBRYOLOGY. 



FIG. 61. Sections of blastocysts of the white rat, showing inversion (entypy) of the germ layers. 

 Huber. A, blastocyst 6 days and 14 hours after insemination. B, blastocyst 8 days and 

 1 8 hours after insemination, with egg-cylinder cut longitudinally. C, similar section of 

 blastocyst 7 days and 22 hours after insemination (younger than B but further advanced 

 in development), showing beginning of proamniotic cavity. D, similar section of blastocyst 

 8 days after insemination (younger than B but further advanced in development), showing 

 more advanced proamniotic cavity. 



i, Ectoplacental cone; 2, ectodermal node; 3, primary embryonic ectoderm; 4, extraembryonic 

 ectoderm; 5, transitory ectoderm (original wall of blastocyst); 6, proamniotic cavity; 

 7, visceral entoderm; 8, cells of parietal entoderm; 9, primary embryonic entoderm. 



