64 EMBEYOLOGY OF THE LOWEE VEETEBEATES gh. 



In the formation of axial mesoderm we recognize a zone of fusion 

 of mesoderm and endoderm accompanied by proliferation of mesoderm 

 entirely analogous with that which occurs in Amphibians but which 

 had not yet made its appearance in lower holoblastie forms. 



Whether it is justifiable to regard the outer zone of mesoderm 

 formation in the Elasmobranch as equivalent to the region of de- 

 lamination (a process which necessarily involves cell-proliferation) is 

 doubtful. It is indeed doubtful to what extent there is justification 

 for drawing any morphological distinction between axial and peri- 

 pheral mesoderm, seeing that the two regions of proliferation are on 

 the protostoma hypothesis morphologically closely related to one 

 another (see Chap. IX.). 



If we look at the matter from the point of view of physiology 

 rather than of morphology we may probably recognize in the active 

 formation of axial mesoderm an expression of the general tendency 

 in the meroblastic egg for all processes of growth and cell prolifera- 



ect. 



mes. 



end. 



Fig. 



39. — Transverse section through the blastoderm of a snake (Tropidonotus) 

 illustrating the origin of the mesoderm. (After 0. Hertwig, 1906.) 



ect, ectoderm ; end, endoderm ; ines, mesoderm. 



tion to become concentrated towards the mesial plane dorsally and 

 to slacken off peripherally and ventrally. 



Ebptiles. — According to the view taken in this book the meso- 

 derm in the holoblastie Craniates at one period spread outwards by a 

 process of delamination from the yolk-laden endoderm. 



In the Amphibians we have seen that a new source of addition to 

 the mesoderm had made its appearance in the form of a zone of pro- 

 liferation on each side of the notochord, in which region cells are 

 budded off into the mesoderm. 



In the Eeptiles — admittedly descendants from Amphibian-like 

 ancestors — in correlation with the concentration of developmental 

 activity towards the mid-dorsal line brought about by the accumula- 

 tion of the yolk ventrally, this parachordal source of mesoderm has 

 become predominant while the lateral source has become greatly 

 reduced. 



In Fig. 39 is represented the typical mode of mesoderm forma- 

 tion as seen in a transverse section through the trunk region of a 

 reptilian embryo. The mesoderm is seen to be spreading out as a 

 wing of cells towards either side from the notochordal or primitive 

 streak region between the two primary cell-layers. 



Birds. — In the Birds also the method of first mesoderm formation 



