GENES AND DEVELOPMENT 145 



nucleus affects the activating substance, but no particular investiga- 

 tions of this point have been made. 



The most important of the regions activated by the substance lies on 

 the dorsal surface of the egg and is known as the differentiation centre. 

 It is also presumably dependent on a cytoplasmic locaUzation. At this 

 centre the formation of the embryonic rudiments begins, and around it 

 the egg is organized as a unitary organism. It is not clear how far one 

 can say that the differentiation centre impresses a pattern on the 

 developing egg or egg-fragment, but it is clear that the centre is as it 

 were the focus roimd which the pattern is formed. Parts of the egg 

 which contain the activator substance but which have been deprived of 

 cellular contact with the differentiation centre can only develop into 

 incomplete structures in which no regulation occurs. 



The occurrence of regulation in fairly late stages of Platycnemis led 

 to a re-examination of the possibility of regulation in other insects. In 

 certain other forms with indeterminate cleavage, regulation by means 

 of a differentiation centre can be shown to occur in the blastoderm 

 stage, as in Platycnemis, or even later. Regulation is certainly much 

 less in determinate types, which include Drosophila. Geigy,^ and also 

 Howland,2 have, however, shown that complete larvae may be obtained 

 from eggs injured immediately after fertilization. Most of the genetic 

 characters studied in Drosophila are characters of the adult, and very 

 Httle is known about their determination. Geigy has reported that after 

 making injuries in young egg stages, he obtained perfect larvae but 

 defective adults, so that it is possible that the adult characters were 

 already determmed and were affected by the injury : on the other hand, 

 injuries to the larvae may have been overlooked. 



5. Echinoderms 



The echinoderm egg is often taken as a nearly perfect example of a 

 regulation egg. But actually regulation of a fragment to give a complete 

 larva is only possible if the egg is cut along a plane parallel to its main 

 animal-vegetative axis. This axis expresses a fundamental cytoplasmic 

 pattern which is present even in the unfertilized egg. The pattern can 

 be considered as two opposed gradients;^ one, having its high end at the 

 animal pole, tends to produce ectoderm and its derivatives, the other 

 with its high end at the vegetative pole, tends to produce an invagina- 



^ Geigy 1931a, h. For Drosophila development see Poulson 1937, Robertson 

 1936. 

 ^ Rowland and Child 1935, Hov/land and Sonnenblick 1936. 

 3 HGrstadius 1928, 1935, 1936. 



