262 Elementary Biology. 



muscular and other layers of the alimentary canal (save the 

 cell-layer lining the canal which is formed by endoderm) ; 

 whilst the outer or parietal layer applies itself to the body- 

 wall, and becomes transformed into a muscular layer. The 

 space between these two layers is the coelom. 



The origin of one other important organ may be alluded 

 to, namely, the notochord. That structure is formed by 

 the folding off of a ridge from the dorsal wall of the 

 enteron. This band afterwards becomes distinct and en- 

 veloped, as does also the nerve-cord, by cells developed 

 from the mesoblastic somites, which become modified to 

 form the fibrous sheath above alluded to. The embryo 

 meanwhile becomes elongated, and the communication be- 

 tween the neural and enteric canals is interrupted, the 

 notochord growing backwards between the two canals. The 

 anterior end of the neural canal now becomes closed, and 

 the walls of the canal thicken to form the spinal cord of the 

 adult. It will be noted that at this time the alimentary 

 canal is a tube closed at both ends. It is put in communica- 

 tion first anteriorly and, later on, posteriorly, by the wall of 

 the tube and the wall of the body growing together, and 

 by the subsequent absorption of the tissue at the point of 

 union. In this way are formed an anterior depression, 

 or stomodseum, and a posterior depression, or procto- 

 daeum (anus). 



In a quite similar way the gill-slits are formed in the 

 pharynx, or anterior portion of the enteron. We have 

 already seen how the atrial folds arise along the sides ,of the 

 body and enclose the atrial cavity. The further changes 

 which take place need not be dealt with. They consist in 

 the gradual moulding of the body of the embryo into the 

 shape it ultimately assumes : the formation of a liver as a 

 diveiticulum from the mesenteron as the alimentary canal 

 between the proctodaeum and stomodaeum is termed and 

 of reproductive and circulatory organs from the mesoblastic 

 somites. 



