I4 NORMAL HISTOLOGY. 



The mesodermic elements, on the contrary, for the most part assume 

 irregular forms and are loosely held together by intercellular substance, thus 



Outer cells 



Zona pellucida 



Inner cells 



Trophoblast 



Trophoblast 

 Zona pellucida 



FIG n Diagrams of very early stages of the mammalian blastodermic vesicle; X.the vesicle con- 

 sists of trophoblast and inner cell-mass ; B, the inner cell-mass is differentiating into ectoderm and ento- 

 derm. (After van Beneden.) 



foreshadowing the features which distinguish many of their derivatives as 

 members of the connective tissue group. 



B 



Embryonic 

 'ectoderm 



Entoderm 



Trophoblast 



Embryonic 

 ectoderm 



Mesoderm 



Entoderm 

 Trophoblast 



FIG. 12. Diagrams of later stages of the mammalian blastodermic vesicle ; A, the thickened embryonic 

 ectoderm corresponds to the area in which the emDryo will develop ; , the mesoderm is appearing as the 

 third germ-layer between the ectoderm and the entoderm, the latter now forming a complete layer. 



The mesoderm undergoes important modifications, splitting into two 

 sheets, a parietal and a visceral layer, between which is included the primitive 



Ectoderm Neural groove Medullary fold 



FIG. 13. Transverse section of rabbit embryo of about eight and one-half days, showing the charactei of 

 the early germ-layers ; the future neural canal is represented by the widely open groove. X 80. 



body-cavity or ccelom. Subsequently this space is subdivided into the great 

 serous sacs of the body the pericardial, the pleural and the peritoneal 



