Experimental Morphology

EXPERIMENTAL MORPHOLOGY The actual structure of any organ in the body of an animal represents a compromise; in its fundamental plan it is determined by the nature of the ancestors of the animal of which it forms a part, while the details are determined by the function which it carries out and by the interference of neighbouring structures. The factors involved are so numerous. and interact so consider ably that no analysis based on a comparison with similar forms can hope to be complete.

The correct scientific procedure in such cases is that of experi ment; one and one only of the factors believed to influence the final structure is changed artificially in a known way, and the resulting changes investigated. Comparatively little work along these lines has been carried out ; the field of work is open and should prove profitable. One aspect of such work has, however, been largely exploited, the investigations commonly grouped to gether under the term experimental embryology.

Explanation of Structure-Changes.--These

studies have had for their object an explanation of the succession of changes in structure which transform a fertilised egg cell into the body of a larva or an adult. To a very large extent they have been con cerned with the earliest part of the process, the cleavage or seg mentation stages, and they have succeeded in demonstrating that for many animals these parts of the whole process of development are pre-determined by the structure of the fertilised ovum, which, apparently uniform in many forms, must itself possess an elaborate and definite structure.

But some investigations, especially those of Speeman and his followers, carried into later stages have shown how in Amphibian embryos the course of development is controlled by definite localised parts of the animal, the "organisers," which take control, one after the other, until some organs become self-determining and can develop into their final structure in isolation. The whole

of this work is strictly morphological in that it is the form and changes of form which are studied.

The structure of an egg, the events which are associated with fertilisation, and the nature of the zygote are investigated by cytologists, whose technical methods have all been designed to make visible in microscopical preparations the architecture of these bodies. The information so acquired is, however, difficult to relate to that which is inferred to exist in the zygote from the evidence afforded by experimental embryology, and only through the very indirect evidence of genetical research has it become established that the chromosomes, the most visible structures in the nucleus, possess a structure of elements, "genes," with a linear arrangement, which is directly associated with a develop mental course which leads to the appearance of structures which exhibit an alternate or Mendelian inheritance.

The investigations of physiologists have made it certain that such elements of an animal's body as muscle and nerve fibres and the surface layer of all cells must possess an ordered arrangement of molecules, whose nature we may hope to discover by the methods of physics and chemistry. Thus in the end the study of biology will involve a morphology, not of organs, tissues or cells, but of molecules and electrons which form the mechanism whose functioning is life.

In philology, morphology is that branch of grammar which examines the form of words as well as the principles of word formation and inflection. In general use the expression morphology has been extended to comprise the general laws of the grammatical structure of a language, i.e., as synonymous with accidence.