22 INTRODUCTION. 



primitive entoderm may also participate in its formation. This 

 third layer is called the mesoderm. Soon after its formation, the 

 mesoderm divides ;it the sides of the embryo into two layers — a 

 parietal, which joins the under surface of the ectoderm, and a vis- 

 ceral, attached to the upper surface of the entoderm. The space 

 between these two layers is occupied by fluid, and is destined to 

 form the future body -cavities. In the axis of the embryo the three 

 earlier layers remain in continuity, forming a cellular mass around 

 the site of the future spinal column (Fig. 4). 



Fig. 4. 



a 



' mend 



Embryo of Necterus in cross-section. (Piatt.) eel., ectoderm : mend., mesoderm ; end., ento- 

 derm ; a, neural groove ; ch, site of future spinal column. 



From these three embryonic layers of cells the body of the foetus 

 is developed. The entoderm, with the visceral or lower layer of the 

 mesoderm, turns downward and inward to meet its fellow of the 

 opposite side and form the alimentary tract. The ectoderm and 

 parietal or upper layer of the mesoderm also turn downward and in- 

 ward, outside of the alimentary tube, and join those of the other side 

 to form the walls of the body. 



Meanwhile, the upper surface of the ectoderm over the axis of the 

 embryo becomes furrowed. The edges of this furrow grow upward, 

 deepening the groove between them, and finally arch over it and 

 coalesce, forming a canal around which the central nervous system 

 i- developed (Fig. 5). Traces of this canal persist through life as 

 the central canal of the spinal cord and the ventricles of the brain. 



The embryonic layers have a deeper significance than the mere 

 furnishing of the architectural materials from which the body is 

 built tip. They are evidence- of a distinct differentiation in the 

 development of the cells of which they are composed. The ecto- 

 derm gives rise to the functional part of the nervous system and the 

 special Benses and to the epithelial structures of the skin and its 

 appendages. The cell- of the mesoderm elaborate the muscular 



