THE vertebrates: the amphibia and birds 



159 



immediately above the blastopore (and thus in the location of the old 

 grey crescent), this develops into notochord; below this region, and 

 further to the side, lie two areas which become the two rows of meso- 

 dermal somites, while the remainder of the belt forms the rest of the 

 mesoderm (side-plate, tail-bud, etc.). Thus, if we look at the map from 

 the blastopore outwards, we fmd first a zone of endoderm within which 

 the young blastopore lies, then a ring of mesoderm, and fmally the ecto- 

 derm. 



The elucidation of the presumptive map in the birds was by no means 

 so simple technically as in Amphibia (summarised: Waddington 1952^). 



Figure 9.6 



Map of the presumptive areas of a urodele gastrula. The blastopore Bl lies 

 in an area of endoderm (in which the gill shts of the pharynx are indicated). 

 Immediately anterior to it is a wide region of presumptive notochord 

 (dotted), and on either side of that somitic mesoderm (lined), which passes 

 off into ventral mesoderm (unshaded). The animal half of the egg is taken 

 up by presumptive neural plate (dashed), and epidermis (unshaded). On the 

 left, seen from the side, on the right from the dorsal surface. (From Pasteels 



1940.) 



A beginning was made by Wetzel in 1929, who was able to stain small 

 regions of the blastoderm in the opened egg; Graper, at about the same 

 time, made speeded-up stereographic cinema films through a window in 

 the shell, from which similar conclusions emerged; and in the next few 

 years, Waddington was able to check many of their suggestions by ex- 

 periments made on blastoderms removed from the shell and cultivated in 

 tissue culture. Improvements of the methods of vital staining were made 

 by Pasteels (1936-7); and Spratt (1946) has developed a technique of 

 marking parts of the blastoderm with carbon particles, which allows of a 

 more precise labelling, but can only be done on embryos cultivated in 

 vitro. 



The results of studies by vital staining in ovo and by carbon marking 

 in vitro have yielded very different maps for the early gastrula, i.e. for the 



