INTRODUCTION. 



23 



tissues and that great group known as the connective tissues, the 

 vascular system, etc., and the entoderm contains the cells that 

 build up the linings of the digestive tract, including its glands, 

 and of the respiratory organs. It appears, then, that this divi- 

 sion of the cells of the embryo into three layers marks a distinct 

 difference in the destinies of the cells composing those layers. 

 This distinction persists through life, the tissues arising from a given 

 layer showing, in general, a closer relationship to each other than 

 the tissues arising from different layers. But this relationship is 

 not always revealed by a similarity in structure, for the latter is 

 determined by the functions the tissues are destined to perform, 

 and tissues of like function acquire a similarity in structure. Thus, 

 for example, the neuroglia in the central nervous system resembles 



Fig. 5. 



Cross-section of fish embryo. (Ziegler.) a, neural canal, cells enclosing it not represented; 

 b, chorda dorsalis, site of future spinal column ; ao, aorta ; Bf, external layer of meso- 

 derm ; c, c, body -cavity ; d, alimentary canal, not yet completely closed * *, passes 

 through the external layer of the mesoderm to its inner surface : e. deutoplasm, or yolk 

 of egg. 



some of the connective tissues, although one develops from the 

 ectoderm and the other from the mesoderm ; and the ganglion cells 

 of the central nervous system differ greatly in structure from the 

 epithelium of the skin, nails, etc., and the cells of the neuroglia, 

 notwithstanding the fact that they all spring from the cells of the 

 ectoderm. The explanation is to be sought in the similarity of the 

 usefulness of neuroglia and connective tissue and the difference in 

 the functions of ganglion cells and those of the other tissues email- 

 ating from the ectoderm. 



During the early stages of development the cells of the germinal 

 layers are very similar in character, although, as we have seen, 



