46 



EMBRYOLOGY 





the two cells are succeeded by four, eight, sixteen, thirty-two, and so on, with the 

 result that a mass of cells is found within the zona striata, and to this mass the term 

 morula is applied (Fig. 9). The segmentation of the mammalian ovum may not 

 take place in the regular sequence of two, four, eight, etc., since one of the two first 

 formed cells may subdivide more rapidly than the other, giving rise to a three- 

 or a five-cell stage. The cells of the morula are at first closely aggregated, but soon 

 they become arranged into an outer or peripheral layer, the trophoblast, which 



Inner cell-mass 



Entoderm 



Blastodermic vesicle 



\ 



Trophoblast 



Fia. 10. Blastodermic vesicle of Vespertilio murinus. (After van Beneden. 



Inner cell-mass Trophoblast 



Embryonic ectoderm Entoderm 

 FIG. 11. Section through embryonic disk of Vespertilio murinus. (After van Beneden.) 



Maternal bloodvessels 



Amniotic cavity 



Syncytiotrcphoblast ' 

 Cytotrcphoblast 



Embryonic ectoderm Entoderm 



FIG. 12. Section through embryonic area of Vespertilio murinus to show the formation of the amniotic cavity. 



(After van Beneden,) 



does not contribute to the formation of the embryo proper, and an inner cell-mass, 

 from which the embryo is developed. Fluid collects between the trophoblast 

 and the greater part of the inner cell-mass, and thus the morula is converted into 

 a vesicle, the blastodermic vesicle (Fig. 10). The inner cell-mass remains in con- 

 tact, however, with the trophoblast at one pole of the ovum; this is named the 

 embryonic pole, since it indicates the situation where the future embryo will be 

 developed. The cells of the. trophoblast become differentiated into two strata: an 



