II.] THE EMBRYONIC APPENDAGES. 29 



blastoderm is spreading both behind (to the left hand in the 

 figure), and in front (to right hand) of the head-fold, its hmits 

 being indicated by the shading and thickening for a certain dis- 

 tance of the margin of the yolk y. As yet there is no fold on the 

 left side of e corresponding to the head-fold on the right. 



^ ia a vertical transverse section of the same period drawn 

 for convenience sake on a larger scale (it should have been made 

 flatter and less curved). It shews that the blastoderm (vertically 

 .shaded) is extending laterally as well as fore and aft, in fact in 

 aU directions ; but there are no lateral folds, and therefore no 

 lateral limits to the body of the embryo as distinguished from 

 the blastoderm. 



Incidentally it shews the formation of the medullary groove 

 by the rising up of the lamina! dorsales. Beneath the section of 

 the groove is seen the rudiment of the notochord. On either side 

 a line indicates the cleavage of the mesoblast just commencing. 



In 0, which represents a vertical longitudinal section of later 

 date, both head-fold (on the right) and tail-fold (on the left) have 

 advanced considerably. The alimentary canal is therefore closed 

 in, both in front and behind, but is in the middle still widely 

 open to the yolk y below. Though the axial parts of the embryo 

 have becoine thickened by growth, the body-walls are still thin ; 

 in them however is seen the cleavage of the mesoblast, and the 

 divergence of the somatopleure and splanchnopleure. The 

 splanohnopleure both at the head and at the tail is folded in to 

 a greater extent than the somatopleure, and forms the still wide 

 sijlanchnic stalk. At the end of the stalk, which is as yet short, 

 it bends outwards again and spreads over the surface of the yolk. 

 The somatopleure, folded in less than the splanchnopleure to 

 form the wider somatic stalk, sooner bends round and runs out- 

 wards again. At a little distance from both the head and the 

 tail it is raised up into a fold, a/, af, that in front of the head 

 being the highest. These are the amniotic folds. Descending from 

 either fold, it speedily joins the splanohnopleure again, and the 

 two, once more united into an uncleft membrane, extend some 

 way downwards over the yolk, the hmit or outer margin of the 

 opaque area not being shewn. All the space between the soma- 

 topleure and the splanohnopleure, pp, is shaded with dots. Close 



