324 THE EVOLUTION OF MAN. 



cavity (c), which has formed between the skin fibrous layer and the intes 

 tinal-fibrous layer. 



Fig. 12. — Longitudinal section through an hypothetical Worm (Chordo- 

 nium), which was among the common parent-forms of Vertebrates and 

 Ascidians. The primitive brain (ju) has lengthened into an elongated spinal 

 tube. Between this spinal tube and the intestinal tube (d), the uotochord 

 (ch) has developed. The intestinal tube has differentiated into two divisions, 

 an anterior gill-intestine (with three pairs of gill-openings, ks) which 

 perves for breathing, and a posterior stomach-intestine (with a liver- 

 appendage, Z6) which serves for digestion. In front, at the head-extremity, 

 an organ of sense [q) has developed. The primitive kidney (u) opens into 

 the body. cavity (c). 



Fig. 13. — Longitudinal section through a Primitive F'sh (Proselachhis) , 

 closely related to the existing Sharks, and hypothetical ancestors of Man 

 (the fins are omitted). The spinal tube has differentiated into the five 

 primitive brain-bladders (m^ — m^) and the spinal man-ow (Wg). (Compare 

 Figs. 15 and 16.) The brain is enclosed in the skull (s), the spinal marruwin 

 the vertebral canal (above the spinal marrow, the vertebral arches (ivh) ; 

 under it the vertebral bodies (wlc) ; under the latter the origin of the ribs is 

 indicated). In front an organ of sense (q, nose or eye) has developed from 

 the horn-layer, — at the back, the primitive kidney (u). The intestinal tube 

 (d) has differentiated into the following parts, lying one behind another : 

 the month-cavity (mh), the throat. cavity with six pairs of gill-openings 

 {ks), the swimming-bladder ( = lungs, lu), the oesophagus {sr), the stomach 

 (mg), the liver (Ih) with the gall-bladder (t). the small intestine (dd), and 

 the rectum with the anus (a). Below the throat-cav.'ty lies the heart, with 

 the auricle (hv) and the ventricle {hk). 



Fig. 14. — Longitudinal section through a human embryo of three weeks, 

 showing tne relation of the intestinal tube to its appendages. In the centre 

 the long-stalked yelk-sac (or the navel-vesicle, ds) projects from the intes- 

 tinal tube (ds) ; similarly the long-stalked allantois (al) projects from the 

 intestine at the back. The heart (hz) is visible beneath the anterior intes- 

 tine. Amnion-cavity (ah). 



Fig. 15. — Longitudinal section through a human embryo of five weeks. 

 (Compare Fig. 14.) The amnion and the placenta, with the urachus, are 

 omitted. The spinal tube has differentiated into the five primitive brain-blad- 

 ders (w^-Wj), and the spinal marrow (w^). (Compare Figs. 13 and 16.) The 

 skull (s) is formed around the brain ; below the spinal marrow the series of 

 vertebral bodies (wk). The intestinal tube has differentiated into the 

 following divisions, lying one behind another : the throat-cavity with three 

 pairs of gill-openings (ks), the lung (lu), the oesophagus (sr), the stomach 

 (rng), the liver (lb), the coil of the small intestine (dd), into which the 

 yelk-sac (ds) opens, the urinary bladder (hh), and the rectum. Heart (hz). 



Fig. 16 — Longitudinal section through developed human female. All 



