24 GENERAL CONCEPTIO.\S. 



3. Spleen. 



4. (?) Tonsils and thymus. 

 Division 2. Sdnguijactive organs. 



i. Bone marrow. 

 Division 3. Genital glands. 



1. Ovary. 



2. Testis. 



The Law of Unequal Growth. 



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

 projections and imaginations 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) enlargement of the cells. In 

 the early stage's 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 funda- 

 mental 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 inter- 

 actions 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 imagination produced; thus for every hair of the skin and 

 for every gland of the intestine there is a separate center of growth. 



The causes of the unequal growths are unknown. We have not even an 

 hypothesis 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 

 no real explanation to say that it is the result of heredity, for that leaves us as 

 completely in the dark as ever as to the physiological factors at work in the devel- 

 oping individual. 



The conception that the development of an animal depends fundamentally 



* By this limitation \vc exclude' the mesenchyma hut not the epithelium. 



