62 THE FIRST DAY. [CHAP. 



that the notochord originates simultaneously with the 

 mesohlast, with which it is at first continuous, as a median 

 plate similarly of hypoblastic origin. 



Kblliker ^ holds that the mesoblast of the region of the em- 

 bryo is derived from a forward growth from the primitive streak. 

 There is no theoretical objection to this view, and we think it woiild 

 be impossible to shew for certain by sections whether or no 

 there is a growth such as he describes ; but such sections as that 

 represented in Pig. 19 (and we have series of such sections from 

 several embryos) appear to us to be conclusive in favour of the 

 view that the mesoblast of the region of the embryo is to a large 

 extent derived from a differentiation of the primitive hypoblast. 

 The mesoblast of the primitive streak forms in part the vascular 

 structures found in the area pellucida, and probably also in part 

 the mesoblast of the aUantois. 



The differentiation of the embryo may be said to 

 commence with the formation of the notochord and the 

 lateral plates of mesoblast. Very shortly after the for- 

 mation of these parts, the axial part of the epiblast 

 above the notochord and in front of the primitive streak, 

 being here somewhat thicker than in the lateral parts, 

 becomes differentiated into a distinct medullary plate, the 

 sides of which form two folds known as the medullary 

 folds, enclosing between them a groove known as the 

 medullary groove. The medullary plate itself consti- 

 tutes that portion of the epiblast which gives rise to the 

 central nervous system. 



Between the 18th to the 20th hour the medullary 

 groove, with its medullary folds or laminae dorsales, is 

 fully established. It then presents the appearance, to- 

 wards the hinder extremity of the embryo, of a shallow 



' Entwick. d. Menschen u. hSlieren Thiere. Leipzig, 1879. 



