38 GENERAL CONCEPTIONS. 



it arose, and connective tissue grows between the two ; the sac may then undergo 

 various modifications. The membranous labyrinth of the ear is developed from 

 the ectoderm in this way, as is also the lens of the eye. We might perhaps also 

 class the medullary canal under this head (cf. Chap. VIII) if we choose to con- 

 sider it as a vesicle so much lengthened that it has become a tube. 



The Law of Unequal Growth. 



The changing shapes of the embryo and the development of the irregulari- 

 ties — projections and invaginations — which preserve the proper proportion 

 between the surface and the mass of the body, both depend upon the unequal 

 growth of the germ-layers, especially in superficies. The expansion of a germ- 

 layer having the epithelial type of structure * may take place by three means : 

 (i) The multiplication of the cells; (2) the flattening out of the cells; (3) en- 

 largement of the cells. In the early stages of development the influence of the 

 first two factors predominates; during the later stages, especially after birth, 

 the latter. Of the three factors, the first is the most important. 



The unequal multiplication of the cells in all embryonic epithelia is the 

 fundamental factor of development, and we see it shaping the embryo, its organs, 

 and the parts of organs, before histological differentiation really begins. The 

 distinct areas and centers of growth which are necessary to develop the human 

 body out of the germ-layers are innumerable, and their distribution, limitations, 

 and interactions make up a large part of the subject-matter of embryology. At 

 every turn of our studies we encounter fresh illustrations. If in a limited area 

 of a cellular membrane there occurs a growth of expansion more rapid than in 

 the neighboring parts, then that area is, as it were, bounded by a fixed ring, and 

 can, therefore, find room for its own expansion only by rising above the level of 

 the membrane; thus, when in the embryonic region of the blastodermic vesicle 

 the growth becomes more rapid, the embryo begins to rise above the level of the 

 vesicle; thus when, at a certain point of the surface of the embryo, a steady and 

 long-continued growth occurs, the limb appears, gradually lengthening out, and 

 enlarges from a small bud at first to a complete arm or leg. If the departure 

 takes place the other way, we have an invagination produced; thus, for every 

 hair of the skin and for every gland of the intestine there is a separate center of 

 growth. 



The reason for the unequal growth is unknown. We have not even an hypo- 

 thesis to offer as to why one group of cells multiplies or expands faster than 

 another group of apparently similar cells close by in the same germ-layer. It is 



By this limitation we exclude the mesenchyma, but not the mesothelium. 



