III.] THE NOTOCHOED. 63 



groove with sloping diverging walls, which embrace be- 

 tween them the front end of the primitive streak. 

 Passing forwards towards what will become the head 

 of the embryo the groove becomes narrower, and deeper 

 with steeper walls. On reaching the head-fold (Fig. 22), 

 which continually becomes more and more prominent, 

 the medullary folds curve round and meet each other in 

 the middle line, so as to form a somewhat rounded end 

 to the groove. In front therefore the canal does not 

 become lost by the gradual flattening and divergence of 

 its walls, as is the case behind, but has a definite termi- 

 nation, the limit being marked by the head-fold. 



In front of the head-fold, quite out of the region of 

 the medullary folds, there is usually another small fold 

 formed earlier than the head-fold, which is the begin- 

 ning of the amnion (Fig. 22). 



The appearance of the embryo and its relation to 

 the surrounding parts are somewhat diagrammatically 

 represented m Fig. 22. The primitive streak now ends 

 with an anterior swelling (not represented in the figure), 

 and is usually somewhat unsymmetrical. In most cases 

 its axis is more nearly continuous with the left, or 

 rarely the right, medullary fold than with the medullary 

 groove. In sections its front end appears as a ridge on 

 one side or rarely in the middle of the floor of the wide 

 medullary groove. 



The general structure of the developing embryo at 

 the present stage is best understood from such a section 

 as that represented in Fig. 21. The medullary groove 

 (m. c.) Uned by thickened epiblast is seen in the median 

 line of the section. Below it is placed the notochord (ch), 

 which at this stage is a mere rod of cells, and on each 



