322 THE EVOLUTION OF MAN. 



tinal-fibrous layer, the first rudiment of the body-cavity, or the caelom (c), is 

 visible ; under it are the two primitive aortas (t). 



Pig. 4. — Transverse section through the germ-disc of a higher Yertebrate, 

 somewhat further developed than in Fig. 3. (Compare the transverse 

 section of the embryo Chick at the third day of incubation, Fig. 95 and 96, 

 p. 317.) The spinal tube (m) and the notochord (ch) are already beginning 

 to be enclosed by the primitive vertebrae (w), in which the muscle-plates, 

 bone-plates, and nerve-roots are becoming distinct. The primitive kidneys 

 (u) are already completely separated from the horn-plate (h) by the leather- 

 plate (I) ; c, the ccelom ; t, the aortas. The skin-layer, rising around 

 the embryo, forms the amnion-fold (am) • this gives rise to a hollow space 

 (#) between the amnion-fold and the wall of the yelk-sac (ds). 



Fig. 5. — Transverse section through the pelvic region and the post3rior 

 limbs of the embryo of a higher Vertebrate. (Compare the transverse 

 section through Chick at the fifth day of incubation, Fig. 120.) The spinal 

 tube (m) is already entirely enclosed by the two curving halves of the 

 vertebrae (wb), and similarly the notochord and its sheath by the two halves 

 of the vertebral body (wit). The leather-plate (I) has entirely separated 

 from the muscle-plate (mp). The horn-plate (h) has thickened very much 

 at the head of the posterior limbs (x). The primitive kidneys (u) are pro. 

 minent in the ccelom (c), and lie very near the germ-epithelium, or the 

 rudimentary sexual glands (k). The intestinal tube (d) is attached to the 

 dorsal surface of the body by the mesentery (g), beneath the main artery (t), 

 and the two principal veins (w). Below, in the centre of the ventral wall, 

 the stalk of the allantois (al) is visible. 



Fig. 6. — Transverse section through a developed Primitive Fish, or some 

 other Vertebrate of a low order. The parts, on the whole, bear the same 

 relation to each other as in the preceding transverse section, Fig. 5, and are 

 marked in the same way. But the sexual glands (k) have developed into 

 ovaries, and the primitive kidneys are transferred into oviducts, which open 

 into the ccelom. The two side protuberances (lb) of the intestinal tube (d) 

 indicate the intestinal glands, for example, the liver. Below the intestinal 

 tnbe, in the intestinal wall, lies the intestinal vein (v) ; above the intestinal 

 tube lies the aorta (t), and above this, again, the two principal veins (n). 



Fig. 7. — Transverse section through one of the higher Worms (through 

 the head of an Annelid), showing its essential agreement with the Verte- 

 brates in the construction of the body from the four secondary germ, 

 layers. It should be carefully compared with the diagrammatic trans- 

 verse section through the low Vertebrate, Fig. 6 : m, the " brain," or "upper 

 throat ganglion." The leather-plate (I) and the muscle-plate, which lies below 

 the former, have differentiated from the skin-fibrous layer. The muscle-layer 

 has separated into an outer circular muscle-stratum and along inner stratum* 

 and the muscle of the latter has distributed itself into dorsal muscles (r) and 

 ventral muscles (6). The two are separated by the primitive kidneys (u), 



