THE vertebrates: the amphibia and birds 169 



der, and a slight depression is visible in the centre of it. The structure has 

 been given the name ofHensens node, and a rather exaggerated importance 

 was attached to it in the earlier literature, since some authors held that it 

 alone was the analogue of the amphibian blastopore; actually, we have 

 seen that mesoderm is invaginated throughout the length of the whole 

 streak. 



Hensen's node does, however, mark the site of a somewhat special type 

 of invagination. In most of the streak during the phase of forward stream- 

 ing, the direction of movement of the mesoderm has been in towards 



Figure 9.12 



The rissue movements in the chick blastoderm : a, during the elongation of 

 the streak; b, in the fully grown streak; c, during the regression of the streak. 

 SoUd arrows show the movements on the upper surface, dotted ones those 

 at a lower level (i.e. in the mesoderm). (From Pasteels 1940.) 



the streak from both sides, and out again in the lower layer towards the 

 sides and somewhat forwards, as indicated in Fig. 9.12. The formation 

 of Hensen's node seems to indicate the beginning of an invagination 

 directed wholly along the midline; tissue, presumptive notochord to be 

 exphcit, moves from slightly anterior to the node backwards into it and 

 after sinking to the lower level, passes out again directly forwards, so that 

 a tongue of notochord extends anteriorly to the tip of the streak. This is 

 known as the head process. 



The formation of the head process brings to an end the forward move- 

 ment along the embryonic axis, and from then onwards all the streaming 

 movements are directed towards the posterior end. Although, as we saw, 



