50 W. B. SCOTT AND HENRY P. OSBORN. 



near the axial line they are crowded together irregularly, 

 but at either side the splitting into two single-celled layers 

 begins to be evident. This splitting begins anteriorly and 

 proceeds slowly backwards. In the posterior sections of the 

 same embryo it is barely evident, although the cells show a 

 tendency to arrange themselves in two rows. Plate IV, fig. 6, 

 represents a section from the trunk region during Stage c, 

 and shows that the splitting of the mesoblast extends slowly 

 backwards. In this region the layer is now thinner than it 

 is forwards, although the reverse of this is true of Stage a, 

 where the mesoblast is thickest posteriorly. The proximal 

 cells now begin to arrange themselves radially around the 

 vertebral portion of the future body cavity, closely im- 

 pinging against the epiblast, and tending to grow in above 

 the primitive notochord. The body cavity does not extend 

 beyond the medullary folds in this embryo, for here the two 

 rows of cells suddenly terminate in a single row bending 

 around the sides. In other respects the mesoblast shows no 

 new features until Stage D. Sections of an embryo, during 

 the latter part of Stage d, show that the neural canal has 

 completely closed. The section figured in Plate IV, fig. 7, 

 is in the anterior trunk region, here the mesoblast appears 

 as two great triangular muscle plates, expanding above so 

 as to fill the space formed by the fusion of the medullary 

 canal, and enclosing the large body cavity. The two layers 

 now extend completely around the embryo, but have not 

 separated except in the upper region. In Stage f the divi- 

 sion into somites has begun. 



To conclude, there is one feature in the development of 

 the mesoblast, which argues strongly for the fact that, meso- 

 blastic invagination being begun, lateral growth sets in at 

 once; that is, the cells formed by invagination are immedi- 

 ately supplemented by those growing down at the sides, of 

 hypoblastic (yolk cell) origin. As evidence of this we find 

 the mesoblast of the posterior sections meeting in the median 

 line below, before it even reaches the ventral region anteriorly. 

 In this single respect, the mesoblast develops more rapidly 

 behind than in front. Subsequent to the formation of the 

 alimentary canal, the greater energy of the embryo is 

 directed to the head region, and all following growth is 

 from before backwards. This is true of the thickening 

 of the lateral plates, of the splitting into two layers, of the 

 formation of the body cavity, and of the subsequent division 

 into somites. 



