CH. V] EARLY DEVELOPMEXT OF THE EMBRYO 



59 



sides have met in front of the medullary folds. The gill-plate 

 is also seen, but the outer medullary folds are no longer con- 

 spicuous. The inner medullary folds are closing in to form 

 a tube. The blastopore is reduced to an elongated slit-like 

 opening. 



A still later stage is drawn in Fig. 20, E. The outline of 

 the whole egg is now elliptical, with the long axis in the direc- 

 tion of the long axis of the embryo. The medullary folds are 

 also much longer, and have approached each other in the 

 middle line. A deep furrow lies between the two halves. The 



FIG. 21. Embryos of Raiia. (After Sclmltze.) Gs, Gs'. Two gill-slits. GP. Gill- 

 plate. SP. Sense-plate. S. Suckers. Ax. Anus. 



folds have more nearly approached at the middle of their 

 length, and are more widely separated at the anterior and 

 posterior ends. At the posterior end the medullary folds are 

 overarching the small elongated blastopore. The sense-plates 

 and the gill-plates are distinctly visible. 



The medullary folds now fuse along their whole length, 

 leaving, as we shall see, a central canal, which is the overarched 

 medullary furrow. The elongation of the embryo continues as 



