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 

 siirnificant. It causes an exchancje 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 Eemak'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, in, 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, n) becomes visible in 



a- 2/ 



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

 Btage 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 defined (a;). (After V'aldeyer.) 



