EXPLANATION OF PLATE V. 323 



which extend from the horn-plate (h) to the ccelom (c). Here the primitive 

 kidneys have a funnel-shaped opening, through which they carry ont the 

 ovules, which fall from the ovaries (k) into the ccelom. The intestinal tube 

 (<I) has glands on its surface (liver-vesicles, Zb). Below it lies the ventral vessel 

 (the intestinal vein, v), above it the dorsal vessel (the aorta, t). The position 

 and origin of all these primitive organs is entirely the same in Man and every 

 other Vertebrate, as in the Worms. The only essential difference is that in 

 the Vertebrates a notochord is developed between the spinal tube and the 

 intestinal tube. 



FIG. 8. Transverse section through the human thorax. The spinal tube 

 (in) is entirely enclosed by the developed circular vertebra (w"). A curved 

 rib proceeds right and left from the vertebra, supporting the wall of the 

 breast (rp~). Below, on the ventral surface, between the right and left rib, 

 lies the breast-bone, or sternum (bb). Without, above the ribs, and the 

 muscles between the ribs, lies the outer skin, formed from the leather-plate 

 (Z) and the horn-plate (h). The greater part of the breast-cavity (or the 

 anterior part of the ccDlom, c) is occupied by the two lungs (Zw), in which 

 the branches of the trachea ramify like a tree. These all open together 

 into the unequal branches of the trachea (), which opens further up 

 at the neck into the oesophagus (sr). Between the intestinal tube and the 

 vertebral column, lies the aorta (t) . Between the trachea and the sternum 

 lies the heart divided by a partition wall into two halves. The left heart 

 (hi) contains only arterial, the right (hr) only venous blood. Each half of 

 the heart is divided by a valved opening into an auricle and a ventricle. 

 The heart is here represented diagrammatically in its (phylogenetic) original 

 symmetrical position (in the centre of the ventral side). In the developed 

 human being, and in apes, the heart lies in an unsymmctrical and oblique 

 position, inclined to the left. 



PLATE V. DIAGRAMMATIC LONGITUDINAL SECTIONS. 



FIG. 9. Longitudinal section through a Gastrula. (Compare Fig. 1, 

 transverse section.) The intestinal cavity (d) opens in front through the 

 mouth (o). The body consists merely of the two primary germ-layers. 



FIG. 10. Longitudinal section through an hypothetical Primitive Worm 

 (Protlielmis) , the entire body of which consists of the four secondary germ- 

 layers. The intestinal tube (d) is still very simple; but the anterior and 

 posterior intestines begin to grow distinct. The mouth (o) is still the anus 

 also. 



FIG. 11. Longitudinal section through a low Ccelomate Worm. The primi- 

 tive brain (m), or the first nerve-centre overlying the throat, has separated 

 from the horn-plate (7i). The intestinal tube has acquired a second posterior 

 anal opening (a) in addition to the mouth-opening (a) in front. A ekin- 

 gland has developed into primitive kidneys (v) and opens into the body- 



