BLASTODEKMIC CELLS. 1 97 



Ordinarily the cells of the Gastriila-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 lirmer 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 earhest process of division or differentiation of the 

 animal body. The germ-membrane (hlastoderrtia), 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, //), a nature differing from 



