How Animals Develop

AbstractEMBRYOLOGY until very recently was almost entirely a descriptive science?a branch of morphology. It dealt with change of form during the progress of development. As is well known, the study received a. great impetus from the theory of organic evolution, for it was in the light of that theory that many of the characteristics of the developmental processes began to become intelligible. Knowledge of embryology in pre-Darwinian days was fragmentary, but it grew enormously as a result of the compelling interest awakened in it by the application of the biogenetic law of development enunciated in slightly different forms by von Baer, Haeckel and others. Stated in its more extravagant and picturesque form, as Milnes Marshall stated it?that an animal in the progress of its development climbs up its own genealogical tree?the law of recapitulation’ is pretty generally rejected by biologists. But to say that an animal during its growth passes through stages which are similar to those which its ancestors passed through in individual development is a statement which no modern biologist would deny. In the light of this law, the theory of organic evolution has given an intelligibility to the facts of development just as it has to many other facts of morphology. But the recapitulation theory does nothing to elucidate the causes of development or of the differentiation which accompanies it. Similarly, until embryology began to be studied experimentally it tended to remain at the descriptive level, like the comparative accounts of the oestrous cycle before the discovery of the chemical substances which are now known to initiate and regulate it.How Animals Develop By C. H. Waddington. Pp. 128 + 7 plates. (London: George Alien and Unwin, Ltd., 1935.) Paper, 3s. 6d. net; cloth, 4s. 6d. net.