Page:Popular Science Monthly Volume 67.djvu/127

Rh the 'Comparative Embryology' he went to Switzerland for recuperation, and met his death, with that of his guide, by slipping from an Alpine height into a chasm. His death occurred in July, 1882. His portrait is shown in Fig. 11.

The memorial edition of his works fills four quarto volumes, but the 'Comparative Embryology' is Balfour's monument, and will give him enduring fame. It is not only a digest of the work of others, but contains, also, general considerations of a far-seeing quality. He saw developmental processes in the light of the hypothesis of organic evolution. His speculations were sufficiently reserved and nearly always luminous. It is significant of the character of this work to say that the speculations contained in the papers of the rank and file of embryological workers, for more than two decades, and often fondly believed to be novel, were for the most part anticipated by Balfour, and also better expressed, with better qualifications.

The reading of ancestral history in the stages of development is such a characteristic feature of the embryological work of Balfour's period that some observations concerning it will now be in place.

Interpretation of the Embryological Record.—Perhaps the most impressive feature of animal development is the series of similar changes through which all pass in the embryo. The higher animals, especially, exhibit all stages of organization from the unicellular fertilized ovum to the fully formed animal so far removed from it. The intermediate changes constitute a long record, the possibility of interpreting which has been a stimulus to its careful examination.

Meckel, in 1821, and later Von Baer, indicated the close similarity between embryonic stages of widely different animals; Yon Baer, indeed, confessed that he was unable to distinguish positively between a reptile, bird and mammalian embryo in certain early stages of growth. In addition to this similarity—which is a constant feature of the embryological record—there is another one that may be equally significant, viz., in the course of embryonic history, sets of rudimentary organs arise and disappear. Rudimentary teeth make their appearance in the embryo of the whalebone whale, but they are transitory and soon disappear without having been of service to the animal. In the embryos of all higher vertebrates, as is well known, gill-clefts and gill-arches, with an appropriate circulation, make their appearance, but