CHAPTER VII 

 THE FINAL ADJUSTMENTS 



The development of an animal is not finished as 

 soon as all its organs have appeared. The details of 

 its structure must still be fitted together and adjusted 

 to one another. Organs like the heart, which are 

 required early in embryonic life, get a start on 

 other less necessary parts and are disproportionately 

 large in the young embryo. The control system of 

 nerves has to be connected up. Particular parts may 

 have to be enlarged to meet special demands made 

 on them. In this chapter some of the ways in which 

 the final ^'tuning" is carried out will be briefly 

 described. 



How Embryos are Fed 



The embryo has to be provided with food until it is 

 old enough to catch and eat its own nourishment. 

 The study of the food-supplies of embryos, and the 

 changes by which the food is digested and then 

 built up into the tissues of the growing animal, is a 

 very complicated one. Until recently the whole sub- 

 ject was a chaos of unrelated facts, but a few years 

 ago Needham brought together all the scattered 

 pieces of information in a work entitled Chemical 

 Embryology and stated the general principles which 

 emerged. In this book we are mainly concerned with 

 the development of animal shape, and there will only 



