THE ASCIDIANS AND AMPHIOXUS 



109 



almost identical with that in Styela, only the peripheral cytoplasm can 

 be distinguished (although it corresponds with the yellowish material in 

 Styela it is grey in Amphioxus). In most other organisms there is no overt 

 sign which distinguishes the regions of different prospective fate, and 

 much ingenuity has had to be used to keep track of the parts of the early 

 egg and discover where they get to and what they develop into. In spite 

 of the technical difficulty of obtaining them, prospective fate maps are 

 the clearest way of summarising the future course of development, and 

 we shall use them again and again to show how the embryological events 



an 



Figure 7.2 



Presumptive areas in the 8-cell stage of an ascidian, seen from the right side. 

 White, ectoderm; a, mesoderm (muscle); b, endoderm; c, notochord; n, 

 spinal cord; n^, brain. (After Vandebroek 1938, Reverberi 1948 and Orto- 



lani.) 



which look so different in different groups of vertebrates, can really be 

 traced back to the same general scheme. 



The prospective fate map of the ascidians, and o£ Amphioxus which is 

 essentially similar, is shown diagrammatically in Fig. 7.2. It has recently 

 been carefully studied by Vanderbroek (see Reverberi 1948) and Ortolani 

 (1954). It will be seen that the yellow crescent o( Styela, which corresponds 

 to the grey crescent which alone is distmgmsh.^^Ac in. Amphioxus, eventually 

 becomes mesoderm. It is thus not the constituent which forms the main 

 body axis, since that is composed of the neural plate with its underlying 

 chorda. The original locations of these two organs are found on the other 

 side of the egg, in the hght grey area of Styela, which does not appear 

 distinctly till after fertilisation. We shall see (p. 146) that in Amphibia it is 



