Ch. VI] FORMATION OF THE GERM-LAYERS 71 



Both assumptions are, I think, erroneous, as a study of the 

 changes that take place in the dorsal lip will convince any one 

 who will take the trouble to follow in the living egg the 

 method by which the closure of the blastopore takes place. 



Later Development of the Mesoderm and Origin of 

 the notochord 



Schultze ('88), who has studied the formation of the middle 

 germ-layer of the frog, has given an accurate account of the 

 condition of the mesoblast in the embryo during the period of 

 overgrowth of the blastopore. He has done this, too, despite 

 the fact that he believes the embryo of the frog to be formed 

 over the upper or black hemisphere of the egg. This belief has 

 not, however, in my opinion, vitiated in any degree his descrip- 

 tion of the position of the mesoblast after its formation. I 

 have, therefore, reproduced his figures in Fig. 26, A-E. 



If a cross-section be made through an embryo (in the plane 

 of the dark line of Fig. 25, A) at the time when the blastopore 

 has assumed a crescentic shape, we find over the surface of 

 the section a thick envelope of ectoderm. The ectoderm is at 

 this time composed of about four layers of cells (Fig. 25, B). 

 In the outermost layer the cells are columnar in shape. In 

 the centre of the section there is a large segmentation-cavity 

 surrounded by large yolk-bearing cells. The archenteron, as 

 seen in cross-section, is a large, arched cavity, its lower wall 

 formed by yolk- cells and its dorsal wall covered by a layer of 

 small cells showing a tendency to become flattened against one 

 another. Above the upper wall of the archenteron, and between 

 it and the ectoderm, is a thick layer of cells. This layer 

 stretches out on each side of the embryo as a lateral sheet, but 

 the edges of the sheet merge insensibly into the yolk-bearing 

 cells at the sides. Where this middle layer (mesoderm) is 

 sharply defined, we can easily distinguish its cells from those 

 of the endoderm, for the mesodermal cells are smaller and pig- 

 mented. At the free edge of the sheet it becomes, however, 

 impossible to distinguish between the cells of the mesoderm 

 and of the endoderm. 



If we examine a complete series of sections through this 



