196 THE FIFTH DAY. [CHAP. 



apex of the heart becomes more marked; the arterial roots 

 are more entirely separated and the various septa completed, 

 so that when the foramen ovale is closed and the blood of 

 the inferior vena cava thereby entirely confined to the 

 right auricle, the heart has practically acquired its adult con- 

 dition. 



10. The fifth day may also be taken as marking the 

 epoch at which histological differentiation first becomes 

 distinctly established. 



It is of course true that long before this date, even from 

 the earliest hours, the cells in each of the three fundamental 

 layers have ceased to be everywhere alike. Nevertheless the 

 changes undergone by the several cells have been few and slight. 

 The cells of epiblastic origin, both those going to form the 

 epidermis and those included in the neural involution, are up 

 to this time simple more or less columnar cells ; they may be 

 seen here elongated, there oval, and in another spot spheroidal ; 

 here closely packed with scanty protoplasm, there scattered 

 with each nucleus well surrounded by cell-substance ; but 

 wherever they are found they may still be recognized as cells 

 of a distinctly epithelial character. So also with the cells 

 of hypoblastic origin, whether simply lining the alimentary 

 canal or taking part in the formation of the compound 

 glands. Even in the mesoblast, which undergoes far more 

 changes than either of the other layers, not only increasing 

 more rapidly in bulk but also serving as the mother tissue 

 for a far greater number of organs, the alterations in the 

 individual cells are, till near upon the fifth day, insignificant. 

 Up to this time, the mesoblast may be spoken of as consisting 

 of little more than indifferent tissue : of nuclei imbedded in 

 a protoplasmic cell-substance. In one spot the nuclei are 

 closely packed together, and the cell-substance scanty and 

 compact ; at another the nuclei are scattered about with 

 spindle-shaped masses of protoplasm attached to each, and 

 there is a large development either of intercellular spaces or 

 of intracellular vacuoles, filled with clear fluid. The proto- 

 plasm differs in various places, chiefly in being more or less 

 granular, and less or more transparent, having as yet under- 

 gone but slight chemical transformation. Up to this epoch 

 (with the exception of the early differentiated blood), there 

 are no distinct tissues, and the rudiments of the various 



