THE DEVELOPMENT OF ORGANISMS 745 



changes of adolescence cannot be separated off from their antece- 

 dents, and thus the broader view of embryology must include the 

 study of the whole organism in its time-relations, even those of 

 senescence. The fine work of Child on Senescence and Rejuvenescence 

 is significant. As we shall notice later on, it is biologically inter- 

 esting to compare the life-histories of different types, for they 

 sometimes differ from one another in the relative length of the 

 various arcs on the life-curve. Thus some, like Peripatus and 

 the elephant, have a prolonged embryonic period; others, like the 

 lamprey, show a lengthened-out larval phase ; others a long adoles- 

 cence, and so on. Some, like eels, die abruptly after reproduction, 

 while others continue parental for many years. Some practically 

 telescope the whole youthful period; others, as in many fishes, 

 continue steadily growing till they come to a violent end. 



EXPERIMENTAL EMBRYOLOGY ILLUSTRATED 



The first task of the embryologist is to describe the development 

 of the individual organism from stage to stage. He tells us by what 

 steps the frog's egg develops into a tadpole, and the tadpole into a 

 frogling. Literally and metaphorically, he takes cross-sections at 

 successive times; and the recombination of these discloses the 

 sequence of individual "becoming". The methods of anatomy and 

 histology afford successive pictures, and these, often photographed, 

 are combined, so to speak, into a film. This is the simplest and oldest 

 kind of embryology, and to emphasise its task of describing a 

 succession of chapters, it may be usefully called Embryography 

 (see section on the Sub-sciences of Biology). 



Gradually, however, though very slowly, the physiological aspect 

 of development became the subject of study. Each stage has to 

 continue the ordinary functioning, e.g. of nutrition and respiration; 

 and the development of one corner of the body is influenced by what 

 is going on in other parts. During each chapter the developing animal 

 has to solve all the fundamental problems of the living creature, 

 except reproduction; though in most cases, save the aquatic, 

 locomotion also remains in abeyance. Moreover, the embryologist's 

 description is very inadequate unless he takes account of the mutual 

 influence of parts (correlation), and the associated harmonising of 

 the development of different parts (regulation). 



Thus each step in the development of an organism is a function 

 of three factors: {a) the specific organisation of the germ-cells, 

 sometimes hinted at by a visible complexity of structure, e.g. in 

 the chromosomes, but with an inconceivable intricacy of molecular 

 complexity beneath this; (b) the vital relations of the various 

 blastomeres or segmentation cells to one another, as they multiply; 



