SECT. 3] AND ORGANISATION 565 



the molecules of which might, by changing either their constitution 

 or their position in space or both, be the underlying factors responsible 

 for the process. An immense field lies open here for the application 

 of micro-estimation methods on sections of tissue dissected out from 

 embryos in the early stages of development, a field which has not 

 so far been entered by one investigator. 



Processes such as chemotropism, moreover, probably have much 

 importance in morphogenesis. This has been emphasised by Kappers 

 whose work on neurobiotaxis has thrown much light on the early 

 development of the nervous system. 



Associated with this conception is that of the localised presence of 

 enzymes. Everything, indeed, depends here on the word "localised". 

 The localisation of substances in the embryo is a study which has 

 not yet begun, for nearly all we know about its physico-chemical- 

 life is concerned with it as a whole and with its immediate sur- 

 roundings. The study of embryonic hormones, for instance, to which 

 much attention will later be given, has not so far been carried on 

 from this point of view, although it does seem clear that the embryo 

 in the early stages is quite devoid of "chemical messengers", and only 

 acquires them later at definite time-points in its development. One 

 may say that the "Nachahmung" school have done much to suggest 

 possible morphogenetic processes without doing anything to find out 

 what they actually are in point of fact. The only means by which this 

 can be done are applications of exact physico-chemical methods, and 

 these will have to be made use of more and more in the future. 



We may, however, before leaving this subject take one instance 

 where chemical work has thrown a light on the state of affairs in 

 the young embryo. In 1926 I studied the free and combined carbo- 

 hydrate contained in the embryonic body of the chick embryo from 

 the 5th day until hatching. Thus 100 gm. of embryo (water and soHd 

 together) contained on the 5th day of development 160 mgm. of total 

 carbohydrate and 8 mgm. of free carbohydrate. During develop- 

 ment the former falls to about 100, and subsequently rises to over 

 300; the latter rises all the time in an S-shaped curve to about 

 48 mgm. This would indicate that the carbohydrate present in the 

 embryo on the 4th and 5th days, at which time there is more in 

 proportion than at any subsequent period, is not in the form of free 

 glucose, and it is certainly not glycogen. The dry weight data gave 

 even more striking results. 100 gm. of dry embryo contain on the 5th 



