i6o 



PRINCIPLES OF EMBRYOLOGY 



Stage at which the streak is just forming. These are shown in Fig, 9.7. The 

 main difference arises from the fact that Pasteels, in his work with vital 

 stains, fmds that during the growth of the streak a considerable movement 

 takes place towards the anterior, while this was not apparent in Spratt's 

 studies. Pasteels' student Malan (1953) has recently examined the matter 

 again, and it has been rather convincingly shown that the absence of this 

 movement in Spratt's material is due to the abnormal conditions of the 

 in vitro culture, to which the early stages are particularly susceptible. Thus 



Figure 9.7 



Maps of presumptive areas in the chick. On the left, just before the streak 

 appears, according to Pasteels, above, and according to Spratt, below (taken 

 from Malan 1953 and Hamilton 1952 respectively). On the right tw^o stages 

 in the formation of the streak (from Waddington 1952^). Epidermis, white; 

 neural tissue, vertical lines; notochord, dotted; axial mesoderm, close 

 horizontal lines ; lateral mesoderm spaced horizontal lines (the very widely 

 spaced lines in Hamilton's map are extra-embryonic mesoderm) ; already in- 

 vaginated mesoderm, crosses. 



although Spratt's map has been accepted by most recent American authors 

 (e.g. Hamilton 1952, Patten 1950, Rudnick 1948), Pasteels' earher one is 

 probably nearer the truth. 



In considering this early stage, we are faced, however, with another 

 uncertainty. What is the presumptive fate of the very early primitive 

 streak? Vital staining has shown that in its later stages the streak consists 

 of presumptive mesoderm. But there are great technical difficulties in 

 making critical experiments of this kind on tlie early stages, and it remains 

 perfectly possible that there is some presumptive endoderm still remaining 

 in the streak when it first forms; this would certainly be so if Jacobson's 



