EXPLANATION OF PLATES IV. AND V. 



' The two Plates IY. and V. exhibit, partly ontogenetically and partly 

 phylogenetically, the mode in which the human body arises from the germ- 

 layers. Plate IV. contains only diagrammatic transverse sections (through 

 the sagittal and transverse axes) ; Plate V. contains only diagrammatic longi- 

 tudinal sections (through the sagittal and longitudinal axes), seen from the 

 left side. The four secondary germ-layers and their products are distin- 

 guished throughout by the same four colours, namely : (1) the skin-sensory 

 layer is orange ; (2) the skin-fibrous layer, blue ; (3) the intestinal-fibrous 

 layer, red ; and (4) the intestinal-glandular layer, green. In all, the letters 

 indicate the same parts. In Fig. 1 and 9 alone the two primary germ- 

 layers are represented — the outer, or skin-layer, orange ; the inner, or 

 intestinal layer, green. In all the figures the dorsal surface of the body is 

 uppermost, the ventral surface underneath. All organs proceeding from 

 the skin-layer are marked with blue letters ; all those proceeding from the 

 intestinal layer, with red letters. 97 



Plate IV. — Diagrammatic Transverse Sections. 



Fig. 1. — Transverse section through the Gastrula. (Compare Fig. 9, 

 longitudinal section, and Figs. 22-28, p. 193.) The whole body is formed by 

 the intestinal tube (d) ; the wall of this consists solely of the two primary 

 germ-layers. 



Fig. 2. — Transverse section through the larva of the Amphioxus, in tha 

 early stage in which the body consists merely of the four secondary germ- 

 layers. The intestinal tube (<£), formed of the intestinal layer, is separated 

 from the body-wall by the coelom (c), which is formed of the skin-layer. 



Fig. 3. — Transverse section through the germ-disc of a higher Vertebrate, 

 with the rudiments of the earliest organs. (Compare the transverse section 

 of the embryo Chick at the second day of incubation, Fig. 92.) The spinal 

 tube (m) and the primitive kidneys (u) are separated from the horn-plate (h). 

 On both sides of the notochord (ch) the primitive vertebrae (uw) and the 

 side-layers are differentiated. Between the skin-fibrous layer and the intes- 



