AVES. 



159 



OT.C 



the lateral parts, becomes differentiated into a distinct medul- 

 lary plate, the sides of which 

 form two folds the medullary 

 folds enclosing between them 

 a medullary groove (fig. 101). 



In front the two medullary 

 folds meet, while posteriorly 

 they thin out and envelop be- 

 tween them the front end of the 

 primitive streak. On the form- 

 ation of the medullary folds 

 the embryo assumes a form not 

 unlike that of the embryos of 

 many Ichthyopsida at a corre- 

 sponding stage. The appear- 

 ance of the embryo, and its re- 

 lation to the surrounding parts 

 is somewhat diagrammatically 

 represented in fig. 102. The 

 primitive streak now ends with 

 an anterior swelling (not repre- 

 sented in the figure), and is 

 usually somewhat unsymmetri- 

 cal. In most cases its axis is 

 more nearly continuous with the 

 left, or sometimes the right, 

 medullary fold than with the 

 medullary groove. In sections 

 its front end appears as a ridge 

 on one side or on the middle of 

 the floor of the widened end of 

 the medullary groove. 



The mesoblast and hypo- 

 blast, within the area pellucida, 

 do not give rise to the whole of these two layers in the surrounding 

 area opaca ; but the whole of the hypoblast of the area opaca, 

 and a large portion of the mesoblast, and possibly even some of 

 the epiblast, take their origin from the peculiar material already 

 spoken of, which forms the germinal wall, and is continuous with 



FIG. 102. SURFACE VIEW OF THE 

 PELLUCID AREA OF A BLASTODERM OF 1 8 



HOURS. 



None of the opaque area is shewn, 

 the pear-shaped outline indicating the 

 limits of the pellucid area. 



At the hinder part of the area is seen 

 the primitive groove pr., with its nearly 

 parallel walls, fading away behind, but 

 curving round and meeting in front so 

 as to form a distinct anterior termination 

 to the groove, about halfway up the pel- 

 lucid area. 



Above the primitive groove is seen 

 the medullary groove m.c., with the me- 

 dullary folds A. These, diverging behind, 

 slope away on either side of the primi- 

 tive groove, while in front they curve 

 round and meet each other close upon 

 a curved line which represents the head- 

 fold. 



The second curved line in front of 

 and concentric with the first is the com- 

 mencing fold of the amnion. 



