36 



FORMATION OF ME SO BLAST. 



an ingrowth from the two sides, but mainly from the formation of 

 cells around the nuclei of the yolk (fig. 16). Shortly after the floor 

 of cells has appeared, the whole segmentation cavity becomes oblite- 

 rated (fig. 17). 



The disappearance of the segmentation cavity corresponds in point 

 of time with the formation of the hypoblast by the pseudo-invagina- 

 tion above described; and is probably due to this pseudo-invagination, 

 in the same way that the disappearance of the segmentation cavity 

 in Amphioxus is due to the true invagination of the hypoblast. 



When the embryonic rim first appears there are no external 

 indications of the embryo as distinguished from the blastoderm, but 

 when it has attained to some importance the position of the embryo 

 becomes marked out by the appearance of a shield-like area extending 

 inwards from the edge of the embryonic rim, and formed of two folds 

 with a groove between them (fig. 28 B, mg), which is deepest at the 

 edge of the blastoderm, and shallows out as it extends inwards. This 

 groove is the medullary groove ; and its termination at the edge of 

 the blastoderm is placed at the hind end of the embryo. 



At about the time of its appearance the mesoblast becomes first 

 definitely established. 



At the edge of the embryonic rim the epiblast and lower layer 

 cells are continuous. Immediately underneath the medullary groove, 

 as is best seen in transverse section (fig. 18), the whole of the lower 

 layer cells become converted into hypoblast, and along this line the 

 columnar hypoblast is in contact with the epiblast above. At the 



sides however this is not the 

 case ; but at the junction of the 

 epiblast and lower layer cells 

 the latter remain imdifferenti- 

 ated. A short way from the 

 edge the lower layer cells be- 

 come divided into two distinct 

 layers, a lower one continuous 

 with the hypoblast in the mid- 

 dle line, and an upper one be- 

 tween this and the epiblast (fig. 

 18 B). The upper layer is the 

 commencement of the mesoblast 

 (?). The mesoblast thus arises 

 as two independent lateral 

 plates, one on each side of the 

 medullary groove, which are con- 

 tinuous behind with the un- 

 differentiated lower layer cells 

 at the edge of the embryonic 

 rim. The mesoblast plates are 

 at first very short, and do not 

 extend to the front end of the 



FlG. 18. Two TRANSVERSE SECTION OF 

 AN EMBRYO OF THE SAME AGE AS FIG. 17. 



A. Anterior section. 



B. Posterior section. 



ing. medullary groove ; ep. epiblast ; hy. 

 hypoblast; n.al. cells formed round the 

 nuclei of the yolk which have entered the 

 hypoblast ; m. mesoblast. 



The sections shew the origin of the 

 mesoblast. 



