roo 



REPORT — 1895. 



until at last in these latter years we investigate not merely the cellular anatomy 

 of the hody, but the anatomy of the cell — if indeed we are permitted to talk of 

 * cell ' at all, and are not rather constrained to express our results in terms of 

 ' cytomicrosomes,' ' somacules,' or ' idiosomes/ and to regard our morphological 

 unit, the cell, as a symbiotic community containing two colonies of totally dis- 

 similar organisms.' To such cytologieal investigations may well be applied Lord 

 Macaulay's aphorism, ' A point which yesterday was invisible is its goal to-day, 

 and will be its starting point to-morrow.' 



Somewhat similar advances in methods have led us from the life-histories 

 studied of old to the new and fascinating science of embryology. The elder Milne- 

 Edwards and Van Beneden knew that in their life-histories Aseldians produced 

 tadpole-like young. Kowalevsky (1866) showed that in their embryonic stages 

 these Ascidian tadpoles have the beginnings of their chief systems of organs 

 formed in essentially the same manner and from the same embryonic layers as in 

 the case of the frog's tadpole or any other typical young vertebrate ; and now we 

 are not content with less than tracing what is called the ' cell-lineage ' of such 

 Ascidian embryos, so as to show the aucestry and descendants, the traditions 

 peculiarities of, and influences at work upon each of the embryonic cells — or areas 

 of protoplasm — throughout many complicated stages. And there is now opening 



EVOLUTION 



Field Naturalist 



MEDICAL 



up from this a great new field of experimental and 'mechanical ' embryology, in 

 ■which we seek the clue to the explanation of particular processes and changes by 

 determining under what conditions they take place, and how they are afl'ected by 

 altered conditions. We are brought face to face with such curious problems as. 

 Why does a frog's ^^^, in the two-celled stage, of which one half has been 

 destroyed, develop into half an embryo when it is kept with one (the black) surface 

 uppermost, and into — not half an embryo, but — a whole embryo of half the usual 

 size if kept with the other (the white) surface upwards. Apparently, according to 

 the conditions of the experiment, we may get half embryos or whole embryos of 

 half size from one of the first two cells of the frog's egg.^ 



One of the most characteristic studies of the older field naturalists, the obser- 

 vation of habits, has now become, under the influence of Darwinism, the ' Biono- 



' See Watase in Wood's Holl Biological Lectures, 1893. 



^ See Morgan, Anat. Anzeig., 1895, t. Bd. p. 623, and recent papers by Rous, 

 Hertwig, Bom, and O. Schultze. 



