THE SCIENCE OF EMBRYOLOGY 



15 



In a few cases, processes which must be considered to be true 'determina- 

 tions', and not mere 'modulations', may later be annulled. For instance, 

 the parts of an early ascidian embryo have only restricted possibilities of 

 differentiation open to them and may be considered to be highly deter- 

 mined; but the adult animal has considerable powers of regeneration, 

 which demand a much greater flexibility than the embryo has at its dis- 

 posal. Such phenomena are not very common in the animal world. They 

 emphasise the fact that, strictly speaking, a given process of determination 

 occurs only in respect of the epigenetic situation at one particular stage 



Mitochond 



Nuclear Membra 

 Nucleolus 

 Fat Droplet 



Fat droplet 

 Vacuole 



Nuclear membrane 



Mitochondria 



Nucleolus 



Figure 1.2 



Nucleolus 



Nuclear membrane 

 Fat droplets and cell granules 

 Mitochondria 



1 in division 

 Centrosphere 



The three basic configurations which cells assume in tissue cukure. Upper 

 left, epithelial; lower left, wandering cells ; right, fibroblasts. (From Willmer 



I954-) 



of development. When we say, for example, that the ectoderm of a newt 

 neurula has been determined either to become neural tissue or to become 

 epidermis, we mean that a choice which was open to it during gastrulation 

 has been settled one way or the other. This need not imply anything about 



