INTERCHANGE OF CELLS BETWEEN THE TWO GERM-LAYERS. 3OI 



tudinal axis of the sole, all three soles coalesce, and here form 

 the thick axial band (Fig. 88, xy). This coalescence is very- 

 significant. It causes an exchange of cells between the 

 primary germ -layers. These cells move, alter their position, 

 and multiply, so that exoderm-cells penetrate among the 

 entoderm-cells, and entoderm-cells among those of the exo- 

 derm. The middle layer, or mesoderm, therefore, contains 

 cells from both of the two primary germ-layers. Even 

 if Remak's explanation, according to which the mesoderm 

 is originally split off from the entoderm, is correct, in 

 consequence of the coalescence at the central point, exo- 

 derm cells may also afterwards make their way into the 

 mesoderm. The fibrous layer indeed soon plainly shows 

 that it is composed of two different strata ; the outer, 

 which, phylogenetically, must be referred to the skin-layer, 

 and the inner, which must be referred to the intestinal 

 layer. The outer is the rudiment of the skin-fibrous layer 

 (Fig. 88, m, 89, m) ; the inner becomes the intestinal-fibrous 

 layer (Fig. 88, /, 89, /). Soon after the coalescence of the 

 germ-layers in the axial portion of the germ-shield has 

 taken place, and the cells have been exchanged, the small 

 rectilineal primitive groove (Fig. 89, ri) becomes visible in 



Fig. 89.— Transverse section through the germ-shield of a Chick (in a 

 stage rather later than in Fig. 88). The letters indicate the same parts as 

 in Fig. 88. In the middle of the axis-band (y) the chorda dorsalis, or noto- 

 chord, becomes denned (x). (After Waldeyer.) 



