BELL-GASTRULA OF AMPHIOXUS. 277 



Fig. 65. — D. The body-cavity appears between the skin-layer and the 

 intestinal layer. 



Fig. 66. — E. The notochord appears between the spinal fiu'row and the 

 intestine. 



Fig. 67. — F. The primitive kidneys and primitive vertebrae appear ; the 

 spinal tube is closed. 



Fig. 68. — G. The rudiments of the sexual organs appear near the primi- 

 tive kidneys. The primitive vertebrae surround the notochord and the 

 spinal tube. 



Fig. 69. — H. The main blood-vessels appear above and below the intestine. 



The letters indicate the same parts in all : d, the intestinal cavity ; del, 

 the intestinal-glandular layer ; df, the intestinal-fibrous layer ; g, mesen- 

 tery; y, female germ-glands (rudimentary ovary) ; x, male germ-glands 

 (rudimentary testes) ; a, aorta (primitive artery) ; vd, intestinal vein 

 (primitive vein) ; vc, cardinal vein ; ch, notochord ; uw, primitive ver- 

 tebrae ; tv, vertebrae ; rm, dorsal muscles ; hm, ventral muscles ; u, primi- 

 tive kidneys ; mf, spinal furrow ; mr, spinal tube ; hs, horn -plate. In all, 

 the four secondary germ-layers are indicated by shading : the intestinal 

 glandular layer (dd) is dotted. The intestinal-fibrous layer (df) is per- 

 pendicularly shaded. The skin-fibrous layer (/(-/) is horizontally shaded. 

 The skin-sensory layer (hs) is b'ack. 



be kenogenetically altered, and we must suppose that 

 the conditions were originally different. This is equally 

 true of the very important germ-history of this lowest Ver- 

 tebrate. In a later chapter (XI Y.) we shall enter into the 

 details of this. Here, however, we may base our argument 

 on this germ-history so far as we are able, from a compara- 

 tive study of the germination of the various Vertebrates, to 

 form an approximate conception of the course of individual 

 evolution, as it originally occurred in the oldest and simplest 

 Vertebrates. Only after we have gained a general view of 

 this, can we turn to the far harder task of tracing the 

 construction of the mammalian organism, and especially 

 that of Man, from the Gastrula. 



The palingenetic Bell-gastrula of the Amphioxus (Fig, 

 28, p. 193) affords a safe starting-point. A series of dia- 



