GENES AND DEVELOPMENT I4I 



appear in a partem, but we cannot even demonstrate any process which 

 has been involved in the determination. 



The concepts which have been summarized above have been worked 

 out mainly on the first formed organs, but they also apply to those 

 which appear later in development. CompUcations enter, however, 

 with the functioning of the blood stream, nerves, etc., which allow 

 reactions to occur between parts of the body which are far removed 

 from each other. In some cases, e.g. hormonal control, it might also be 

 possible, though perhaps unnecessary, to use the analysis into evocators 

 and competent tissues. If the tissue responds to a hormonal stimulus by 

 any particular pattern of reaction, this pattern is presumably a property 

 of the tissue itself, i.e. of the competence rather than of the stimulus. 

 We shall see that even in early development the competent tissues may 

 not be completely without pattern properties. 



After this general account, which has attempted to show the funda- 

 mental similarities of developmental processes throughout the animal 

 kingdom, it will be as well to exhibit the variations found in different 

 groups and their dependence on genetic factors. 



3. Mosaic Eggs 



The eggs of Ascidians^ may be considered as the best-known example 

 of mosaic eggs, though even in these recent work has shown that some 

 regulation is possible in very early stages. In Styela, Conklin found that 

 at least three organ-forming substances are present before fertilization ; 

 a clear cytoplasm surrounding the egg nucleus, a yellow peripheral 

 cytoplasm and a grey inner mass of cytoplasm. These regions cannot of 

 course be formed under the influence of the zygote nucleus, since they 

 are present before it is constituted. Before fertiUzation they are already 

 arranged in a pattern which is related to the primary egg axis defined 

 by the position of the egg nucleus and the polar bodies. The fertiHzing 

 sperm enters in a restricted region near the vegetative pole, and the 

 cytoplasmic regions then move, in a definite way related to the move- 

 ments of the nuclei, and also break up into further demarcated regions, 

 which, by the time the nuclei fuse, lie in the fundamental pattern of 

 the vertebrate egg which we shall meet again in the Amphibian blastula. 



Although these movements are related to the two parental nuclei, 



there is no evidence that the relation is specific for the particular 



genetic constitution of the nuclei, which act as triggers releasing a 



reaction rather than as determinants of the course of events. In any 



1 Conklin 1905, 1924, Dalcq 19323 I935^- 



