564 ON INCREASE IN COMPLEXITY [pt. iii 



General increase of thickness and extent is among the simpler 

 problems, and seems to be but a sub-section of the general question 

 of growth. All the discussion of this difficult subject is complicated 

 by the fact that nearly everything that has been ascertained about 

 the physico-chemical constitution and activity of the embryo has 

 been ascertained on the intact organism. It is only in very recent 

 times that the first step has been taken towards the penetration of 

 the internal balance of the embryo, namely, the evaluation of the 

 growth-rates and relative sizes at different times of its constituent 

 organs. But surer knowledge about this is bound to come in the 

 not too distant future. Meanwhile it is simple to picture the processes 

 of perforation, for example, as being due to local controlled autolysis. 

 Here the importance of local variations of intracellular pB. must not 

 be forgotten, and a study of the intracellular p¥L in a great variety 

 of regions at different times during development would be a most 

 valuable work, especially if it were to be done with methods much 

 better than those which have up to now been employed in the 

 estimation of intracellular pH. Buytendijk & Woerdemann have 

 made a good start in this direction (see Section 6- 1 ) , and there are other 

 investigations which have a similar theoretical importance, but they 

 will be referred to in their place. The powerful influence of />H on 

 cell-division, as in the work of Reding & Slosse and of Vies, Dragoiu 

 & Rose may, liowever, be mentioned in this connection. The two 

 tables should be borne in mind throughout all succeeding chemical 

 discussions. 



Just as perforations may be probably due to local controlled 

 autolyses, so general disappearances of masses of cells may be due 

 to the same factor operating on a slightly larger scale. This con- 

 ception is made all the more Ukely by the work which has been 

 done since 191 9 on insect metamorphosis, and which goes to show 

 that there also an autolysis plays a large part. Again, the changes 

 in the form and size of individual cells which may grow larger or 

 smaller may probably depend on the increasing or decreasing con- 

 centration of storage materials in them, and here there is an extra- 

 ordinarily wide field for histochemical methods. Unfortunately these 

 are at present so unsatisfactory that it may be a long time before this 

 aspect of morphogenesis can be even approached. The separation of 

 cells and their fusion into surfaces and layers may also be due to 

 local variations in the sterol and lipoid fractions (tissue constants), 



