FOKMATION OF ENTODEKM 



does not grow out round the wall of the vesicle closely applied to the ectoderm 

 (see fig. 41), but speedily forms a small closed sac, the entodermic or future 

 yolk-sac (fig. 39, b and c), separated by a space from the trophoblastic wall 



FlG. 38. DlAGBAM TO SHOW THE DIFFERENCE BETWEEN THE BLASTOCYST (BLASTULA) OF A 

 PLACENTAL MAMMAL a, AND THAT OF ONE OF THE LOWEB AMNIOTA b. (After Semon.) 



In a, the wall of the blastocyst is complete from the first, and the formative cell-mass projects 

 into its interior ; in b, the wall is completed only at a much later stage by the growth of the 

 ectoderm over the yolk, and the formative cells spread out on the surface. 



of the blastocyst. This peculiarity is clearly secondary, and is due to the 

 precocious and extensive expansion of the trophoblast shell, while the formative 

 cell-mass lags behind in development. The entoderm layer, which clings to the 



fcm. 



rfr 



mb. ect. 



FIG. 39. EARLY STAGES IN THE FORMATION OF THE GERMINAL LAYERS IN TABSIUS SPECTKUM. 



(After Hubrecht.) 



fcm., formative cell-mass ; ent.^ entoderm ; emb. ect., embryonic ectoderm. 



embryonal cell-mass, is composed, as in many lower mammalian forms, of larger 

 more loosely arranged cells, while the free portion is formed of more flattened 

 elements. 



It is not quite clear how the layer of entoderm cells becomes converted into a sac. There are 

 three possibilities : 1. The cell-layer grows meridionally in all directions, finally to close in 



