CONCRESCENCE. 



243 



as a means of absorbing nutrition. In the vertebrates, tunicates, enteropneusta, 

 amphioxus, and pterobranchia the cephalic fistula became a permanent opening 

 into the enteron, thus giving rise to a new oral opening, or hsemostoma, at the 

 time when the old mouth was about to close up. 



V. Concrescence and the Caudal Navel or "Blastopore." 



We have seen that the body of segmented animals consists of an axial cord 

 and a right and left series of segments, or half metameres, and that the body 



Fig. 151. — Same. Embryo just before it escapes from the inner egg membrane, stage 0. The cephalic 

 navel has closed and the degenerating remnants of the haemal blastoderm are seen enclosed in the first four pairs of 

 liver lobes, giving them a mottled appearance. The lobes are still partly filled with yolk. 



grows in length by the formation of new segments at the anal plate, and in breadth 

 by the increase in length of the half metameres. But as each half metamere 

 grows older and longer its peripheral end increases in width faster than its central 

 end. Hence, as new segments are successively formed, the lateral margins of the 

 germinal area increase in length faster than the axial, and the germinal area, in- 

 stead of forming an elongated band of nearly uniform width, or a triangle, forms 

 first a heart-shaped figure, and finally one in which the peripheral margins con- 

 cresce in front of and behind the main axis. (Fig. 157, A.C.) When this has 



