SECTION 2 

 ON INCREASE IN SIZE AND WEIGHT 



2-1. Introduction 



We have so far been considering the unfertilised egg-cell and its 

 reserves of nutrient material as a physico-chemical system, and we 

 must now proceed to summarise critically what is known about 

 the alteration the egg undergoes in passing into the state of the 

 finished embryo. Subsequent sections will take up the chemical 

 changes during this process in all their complexity, but first the 

 apparently simple phenomena of change of weight must receive con- 

 sideration. To this undertaking special difficulties are attached; for 

 example, the act of birth or hatching itself, important though it is 

 for the chemical embryologist as the term of his investigations, is yet 

 purely arbitrarily and conventionally chosen as such, and, as far as 

 the organism itself is concerned, may be relatively unimportant. The 

 age at which birth takes place varies in different animals consider- 

 ably, and may occur earlier or later in development, cutting across 

 cycles of growth at almost any point. However, the study of growth 

 in weight and alteration in shape, is an essential preliminary to the 

 study of the chemistry of the embryo. I do not propose to spend 

 any time in the discussion of definitions of growth. 



The actual data which we have concerning pre-natal growth 

 will be found in Appendix i, where they have been placed in 

 the hope that a collection of them will be of assistance to chemical 

 embryologists. No previous assemblage of them has been made, and 

 they are to be found scattered all through the literature. Biochemists 

 have in the past been insufficiently careful to check their results on 

 embryos against normal tables of weight, length, age, etc. 



The predecessors of this section are the chapters on growth in 

 d'Arcy Thompson's Growth and Form and Faure-Fremiet's La Cinetique 

 du Developpement. These authors gave a full criticism of the whole 

 subject, but without special reference to the development of the 

 embryo. Moreover, much has been done since they wrote, and their 

 treatment differs in various ways from what follows here. 



