12 INTRODUCTORY I 



Thus he suggests that the migrations of vitellophags and 

 mesenchyme cells, the thickenings, thinnings, and perforations 

 of flat layers, the rearrangements of cells in a massive aggregate, 

 their dispersion, the constriction, and splitting and fusion may 

 be regarded as tactic responses, the growth in various ways of 

 linear aggregates, the concrescence of layers and masses as so 

 many tropic responses to stimuli which may be positive or negative 

 and exerted by other organs or by agents in the world outside. 



Now it is clear that the analyses both of His and Davenport 

 aim at something more than a mere description of ontogenetic 

 events, for a serious attempt is here made to give a causal, if 

 you will a mechanical, explanation of those events, and the sub- 

 ject thereby raised at once from the level of mere morphology or 

 morphography to a loftier, aetiological point of view. 



There are, indeed, two methods by which embryology, like 

 any other branch of zoology, may be investigated. One is 

 purely descriptive, anatomical, morphological. By this method, 

 truly, great results have been achieved. The life-histories of 

 members of all the most important groups of the animal king- 

 dom have been worked out, and the science of Comparative 

 Embryology has been built up. Nor has an explanation of the 

 process been lacking. For ontogeny is, the fundamental Bio- 

 genetic Law assures us, a recapitulation of and therefore explicable 

 in terms of phylogeny ; and since on this principle the individual 

 repeats in its development the ancestry of its race, embryology 

 affords a means of tracing out the relationships of the organism 

 and establishing the homologies of its parts. 



Unfortunately a more intimate acquaintance with the facts 

 has made it abundantly clear that development is no mere repeti- 

 tion of the ancestral series, that the organism has manifold ways 

 of attaining its single end, that those resemblances in early stages 

 which were held to constitute the most triumphant vindication of 

 the Biogenetic Law bear no constant relation to the similarities 

 of adult organization, that the attempt to find in development 

 an absolute criterion of homology is vain. 



The facts thus remain unexplained, as in truth it was only to 

 be supposed that they would. A method, however comparative, 

 which relies on mere observation, and is content to wait for 



