158 



PRINCIPLES OF EMBRYOLOGY 



This lies iii the maps of presumptive^ fate; if we take the centres of 

 gastrulation as our points of reference, these have a similarity which 

 the crude comparison of the blastula and the blastoderm does not exhibit. 

 Amphibian eggs were the first to which the technique of vital marking 

 was systematically applied to discover the map of presumptive area. The 

 method consists in pressing against some region of the egg a small lump 

 of agar impregnated with a dye which will colour the cells without doing 

 them any great damage. The coloured patch can then be followed through 



Figure 9.5 



The movement of vitally stained marks towards the blastopore, and thence 

 into the mesoderm ; e shows the location of the material in a transverse sec- 

 tion of the neural plate stage. (From Vogt 1929.) 



its later development, and its fate ascertained (Fig. 9.5). This was done by 

 the German embryologist W. Vogt in 1925, and his results, summarised 

 in 1929, have remained substantially unchanged ever since, although 

 many later authors hc^^^ i^ vised them in detail (Fig. 9.6). 



Vogt showed that '^^7 whole lower part of the blastula becomes 

 endoderm and forms tB^<^'ut and its annexes ; the whole upper hemisphere 

 becomes ectoderm, diffe Vitiating into the nervous system and the epider- 

 mis with its derivatives ^uch as the ears, nasal placodes, etc. In between 

 these two lies a broad belt w^ -^h forms the mesoderm; on the dorsal side, 



^m.. 



^ The word used by the German authors who originally made such maps was *pra- 

 sumptive', and many English authors use 'presumptive' for this: American writers, 

 however, tend to use 'prospective*. 



