1888.] Prof. Wilhelm His on Animal Morphology. 293 
displacement of its masses. By further inquiries we find that, 
independently of an increase of volume, the growth of the germinal 
surfaces is the most general feature of this period, and that it is the 
cause of the different foldings which precede the formation of an 
embryo. The body of an osseous or an elasmohranch fish arises 
by the coalescence of two halves previously formed at the periphery 
of the germinal disc. This fact is also proved by measuring the 
different parts of the germ during the process of formation. 
The ways of determining the forms and volumes of germs and 
embryos are somewhat longer and more tiresome than the simple 
inspection of stained sections ; hut the general scientific methods of 
measuring, of weighing, or of determining volumes cannot be 
neglected in embryological work, if it is to have a solid foundation 
of facts, for morphologists have not the privilege of walking in 
easier or more [direct paths than workers in other branches of 
natural science. 
But we must go farther in our propositions. Embryology and 
morphology cannot proceed independently of all reference to the 
general laws of matter, — to the laws of physics and of mechanics. 
This proposition would, perhaps, seem indisputable to every natural 
philosopher; hut, in morphological schools, there are very few who 
are disposed to adopt it with all its consequences. 
Twenty years ago I worked on the development of the chick. 
Starting from histological questions, I came to follow the formation 
of the body from the primitive germinal layers, and by-and-hye my 
attention was fixed upon different relations of an evidently mechanical 
kind. I found, for instance, that the enlargement of the medullary 
tube was always coincident with some flexure of its axis, and that 
different degrees of inflection corresponded to different degrees of 
enlargement, &c. 
These first empirical observations led me farther. Folds of the 
primitive layers determine the limits of the embryonic body, the 
limits of the right and the left side, the limits of the head and of 
the trunk with its segments. Eolds also form the nervous centres, 
the heart and the intestine. The principle of layer-folding is there- 
fore a fundamental one in embryology, and the study of its conse- 
quences must be one of the most important tasks of this science. 
The germinal layers are elastic plates under the influence of certain 
