THE SCIENCE OF EMBRYOLOGY 5 



able deficiency, our lack of knowledge of exactly what genes do and how 

 they interact with other parts of the cell in doing it. But whatever the 

 immediate operations of genes turn out to be, they most certainly belong 

 to the category of developmental processes and thus belong to the pro- 

 vince of embryology. This central problem of fundamental biology at 

 the present time is of course being attacked from many sides, both by 

 physiologists and biochemists and by geneticists; but it is essentially an 

 embryological problem. 



It is unlikely that the methods of classical descriptive or experimental 

 embryology will suffice to bring any solution to the problem of the 

 genetic control of development. Neither will the conventional breeding 

 methods of classical genetics, or, in all probability, the normal techniques 

 of biochemistry and physiology. A general textbook of embryology can, 

 however, not be confined to those novel techniques of investigation which, 

 at any given time, seem most likely to lead to major advances in under- 

 standing. New methods can usually only be appHed to old material; and 

 new ideas do not suddenly emerge full-fashioned, as Aphrodite was born 

 from the chaotic sea; they are built up laboriously on the foundation of 

 previous work. Thus this book will attempt to describe, in the abbreviated 

 and simphfied outline which considerations of space impose, the general 

 framework of embryological science within which the attack on the 

 fundamental problems has to be made. Those problems cannot always be 

 in the forefront, but the importance of the various aspects of embryology 

 will be better appreciated if one has a clear realisation of the nature of the 

 goal towards which our expanding knowledge is advancing. 



2. An outline of development 



Since all animals are in some way related, through the processes of 

 evolution, there are some similarities in their various forms of develop- 

 ment. One can, in fact, sketch a broad outline of the early stages of devel- 

 opment which apphes, roughly at least, to all the animal phyla. This can 

 best be described in terms of a series of stages : 



Stage I. The maturation of the egg. The period during which the egg-cell 

 is formed in the ovary might be thought to come, as it were, before em- 

 bryology begins, but actually it is of great importance. It is, of course, 

 the time when the meiotic divisions of the nucleus occur and the number 

 of chromosomes is reduced to the haploid set. Further, the egg is pumped 

 full of nutritive materials of various kinds, collectively known as 'yolk' 

 (in the broad sense of that word); there are usually special 'nurse cells', 

 closely apphed to the growing egg in the ovary, which are concerned in 



