598 E. W. MACJmiDE. 



this confirms the ideas of Balfour^), the bulky ventral wall of 

 the archenteron can no longer be folded in, and the persistent 

 invagination of the yolkless dorsal wall has the appearance of 

 an independent ingrowth of the ectoderm. 



A good illustration of the influence of yolk is seen in the 

 development of Molluscs. There it has been customary to 

 regard the four macromeres as representing the endoderm. 

 These cells are, however, much too heavily loaded with yolk 

 to give rise to any definitive tissue ; from them the smaller 

 micromeres which are to form the ectoderm are budded oflp, 

 and from them later, in continuity with these, the cells which 

 give rise to the epithelium of the intestine. (See Balfour's 

 ' Text-book of Comparative Embryology/ vol. i, Dev. of 

 MoUusca.) It is significant that assertions of the share of the 

 ectoderm in the formation of the alimentary canal should have 

 been made principally in such cases (eggs of insects, cephalo- 

 pods, &c.), where the accumulation of yolk is so great as to 

 preclude any possibility of the yolk-bearing cells being directly 

 converted into permanent tissue. 



2. The Formation of the Mesoderm. 

 Shortly after the gastrulation is complete the outline of the 

 alimentary canal, as seen in transverse section, ceases to be 

 round. Its dorsal wall becomes flattened, and is then drawn 

 out into two lateral angles, and almost immediately afterwards 

 a median hollow ridge is formed, the first suggestion of the 

 future notochord. The lateral angles mentioned are the ex- 

 pression of two longitudinal hollow ridges or folds of the gut 

 wall. Lwoff regarded them as the mechanical efi'ect of the 

 downward pressure of the nerve-tube as it became folded into 

 the interior of the body, and likewise of the notochord when it 

 was formed. The absurdity of this view is well shown by 

 PI. 43, fig. 12 ; there these two lateral ridges are plainly seen, 

 whilst the nerve-cord is still a flat plate, and the notochord is 

 barely indicated. That these hollow outgrowths (which we 



^ "A Comparison of the Early Stages in the Development of Verte- 

 brates," bj r. M. Balfour, ' Quart. Journ. Mic. Sci.,' 1875. 



