EXODERM AND EXTODERM CELLS. 211 



germ-layers. The brighter and harder cleavage-cell (Fig. 

 38, e) is the mother-cell of the exoderm ; the darker and 

 softer cleavage-cell (Fig. 38, i) is the mother-cell of the 

 entoderm. All the cells of the outer germ -layer, the skin- 

 layer, are produced from the exoderm mother-cell (Fig. 

 38, e; Plate II. Fig. 13, e). In the same way the whole of 

 the cells of the inner germ-layer, the intestinal layer, 

 descend from the entoderm mother-cell (Fig. 38, i\ Plate 

 II. Fig. 13, i). This interesting relation, which we thus see 

 in the mammalian germ, is yet more pronounced in the 

 germs of many lower animals. In many Worms, for 

 example, at the beginning of cleavage, the parent-cell 

 parts into two cleavage-cells of very dissimilar size and 

 chemical qualities. In such cases the mother-cell of the 

 exoderm is often very many times smaller than the ento- 

 derm mother-cell, which contains a large store of nutritive 



yelk. 



The two first cleavage-cells of the Mammal, which are 

 to be regarded as the mother-cells of the two primary germ- 

 layers, now contemporaneously separate into two cells (Fig. 

 39 ; Plate II. Fig. 14). These four cleavage-cells usually lie 

 in two different planes, perpendicular to each other; more 

 rarely in one plane. The two larger and brighter cells 

 (Fig. 39, e), the descendants in the first generation of the 

 exoderm mother-cell, if placed in carmine, colour much 

 more deeply than do the two smaller and darker cells, the 

 descendants of the entoderm mother-cells (Fig. 39, i). The 

 line which connects the central points of the two latter 

 cleavage-globules is usually perpendicular to that which 

 connects the central points of the two latter. Presently 

 each of these four cells again divides into two similar cells ; 



