VERMIXAL WALL. 



A 



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 between them the front end of the primitive 

 streak. On the formation of the medullary folds the embryo assumes 

 a form not unlike that of the embryos of many Ichthyopsida at a 

 corresponding stage. The appearance of the embryo, and its relation 

 to the surrounding parts is somewhat diagrammatically represented in 



fig. 102. 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 

 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 hypoblast, 

 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 hy- 

 poblast of the area opaca, and a 

 huge 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 the hypoblast at 

 the edge of the area opaca (vide 

 figs. 91, 94, 97, 98, 99, 100). 



The exact nature of this material 

 has been the subject of many con- 

 troversies. Into these controversies 

 it is not my purpose to enter, but 

 subjoined are the results of my own 

 examination. The germinal wall first 

 consists, as already mentioned, of the 

 lower cells of the thickened edge of 

 the blastoderm, and of the subjacent 

 yolk material with nuclei. During 

 the period before the formation of the 

 primitive streak the epiblast extends 

 itself over the yolk, partly, it appeal's, 

 at the expense of the cells of the 



FlG. 102. SuiiFACE VIEW OF THE 

 PELLUCID AKEA OF A BLASTODERM OF 18 



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 be- 

 hind, but curving round and meeting 

 in front so as to form a distinct an- 

 terior termination to the groove, about 

 halfway up the pellucid area. 



Above the primitive groove is seen 

 1h" medullary groove m.r., 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 

 commencing fold of the ainnion. 



