8 THE NEW CELL DOCTRINE 



man and many others have greatly advanced the recogni- 

 tion of the actual relations. We know now that when an 

 ovum begins its development it must be regarded as a 

 complete cell. This cell divides, the process being usually 

 termed the segmentation of the ovum. When the ovum 

 divides there arise two new cells which then divide again. 

 If we investigate the relations of such cells in vertebrates 

 we may observe without difficulty that the cells are com- 

 pletely isolated from one another. They have no direct 

 communication between themselves. They live alongside 

 one another, but the living substance of one cell is nowise 

 united with the living substances of the neighbor cell. In the 

 course of the further development, however, the relation 

 changes because the cells begin to unite with one another. 

 This occurs chiefly in two ways. Consequently we obtain 

 two kinds of tissues which we regard as the primitive tissues of 

 the body, since from them all the tissues of the adult are slowly 



Ba 



FIG. 5. Epithelium (epidermis) of a chicken embryo of the second day of incu- 

 bation. The nuclei are mostly oval and lie scattered. The protoplasm forms a 

 network. There are no intercellular partitions present. Eph, epitrichial layer; 

 Ba, basal layer. 



differentiated. In one form we find the cells completely fused 

 with one another and they build a continuous layer which we 

 designate as epithelium. In such a primitive epithelium, Fig. 

 5, there are no limits between the single cells, but on the 

 contrary one has a continuous layer of protoplasm in which 

 the nuclei are scattered, though generally rather close to- 



