i GASTEULATION 53 



been eliminated entirely from ontogenetic development in Birds. 

 What is conspicuous is a well-marked primitive streak which 

 makes its appearance in the posterior half of the blastoderm along 

 what will be the axial line of the body of the embryo (see Chap. X.). 

 A groove develops along the surface of the streak — the primitive 

 groove. 



Histologically the primitive streak is, in its early s,tages, a line 

 of proliferation from the inner surface of the ectoderm, the blasto- 

 derm being composed only of the two primary layers at the time 

 of its appearance. That the ectoderm alone is responsible for the 

 first appearance of the primitive streak, a point difficult to make 

 absolutely certain by ordinary observation, appears to be demonstrated 

 by the study of an abnormal 36 -hour embryo Peawit {Vanellus 

 cristatus) described by Eothig (1907) in which the endoderm was 

 completely absent while ectoderm and primitive streak were 

 quite normal. 



An inspection of blastoderms at successive periods in development 

 shows the primitive streak lying always behind the medullary folds 

 (cf. Fig. 227, Chap. X.), and it might therefore be readily assumed 

 that the embryonic body develops entirely in front of the primitive 

 streak. That this is not so is clearly shown by experiments (Kopsch, 

 1902) in which a scar is made with a hot needle about the front 

 end of the primitive streak during an early stage in its development. 

 If the egg is carefully sealed up again it may go on with its 

 development, and in such a case the scar is found later on to be 

 situated not near the hind end of the embryo but well forward in 

 the head region. 



What apparently happens is that the primitive streak grows 

 actively in length with the general growth of the ' blastoderm but 

 that all the while it is becoming correspondingly shortened at its 

 headward end. As a matter of fact its anterior end becomes 

 gradually converted from before backwards into notochord and the 

 adjoining parts of the mesoderm. The front part, which is under- 

 going this change, loses its connexion with the ectoderm while it 

 becomes on the other hand continuous with the endoderm and is 

 reinforced by proliferation from it : it then forms what is known as 

 the Head process. 



The point of special morphological importance to notice about 

 the primitive streak is its continuity with the two primary cell- 

 layers. Throughout the greater part of its length it is continuous 

 with ectoderm, in its front half with both ectoderm and endoderm, 

 and in its forward prolongation — the head process — with endo- 

 derm. Correlated with this is the further fact that in some cases 

 (Tern, Goose, Duck, Wagtail, Melopsittacus) the tissue of the primitive 

 streak is traversed by a typical neurenteric canal. 



Taking these various features into consideration it is impossible 

 to avoid the conclusion that the primitive streak represents the 

 line of coalescence of the gastrular lips just as it actually is in 



