BLASTODERMIC CELLS. 1 97 



Ordinarily the cells of the Gastrula-germ, which com- 

 pose the two primary germ-layers, already present recog- 

 nizable differences. In most cases, if not in all, the cells 

 of the skin-layer, or exoderm (Fig. 29, e), are smaller, more 

 numerous, and brighter coloured; on the other hand, the 

 cells of the intestinal layer, or entoderm (Fig. 29, i}, are 

 larger, less numerous, and darker. The protoplasm of the 

 exoderm cells is clearer and firmer than the darker and 

 softer cell-substance of the entoderm cells; the latter are 

 generally much richer than the former in fatty particles. 

 The cells of the intestinal layer usually also have a much 

 greater affinity for colouring matter, and take up carmine, 

 aniline, and so on, from solution much more quickly and 

 vigorously than do the cells of the skin-layer. 



These physical, chemical, and morphological differences 

 in the two germ-layers correspond to their physiological dif- 

 ferences, and are of great interest, because in them we see the 

 first and earliest process of division or differentiation of the 

 animal body. The germ-membrane (blastoderma), which 

 forms the wall of ' the globular germ-vesicle, or Blastula 

 (Fig. 22, F, G), consisted solely of a single layer of similar 

 cells. These cells of the germ-membrane, or blastoderm, are 

 usually formed in a very regular and even way, and are of 

 entirely similar size, form, and qualities. Generally they 

 are flattened by pressing against each other, and are often 

 uniformly six-sided. This uniformity of the cells disap- 

 pears, at an earlier or later period, during the inversion 

 (invaginatio) of the germ-vesicle. The cells, composing 

 the inverted, inner part of the germ-vesicle (which after- 

 wards form the entoderm) usually assume, even during the 

 process of inversion (Fig. 22, IT), a nature differing from 



