DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 



113 



Fig. 73. — Diagram to show the development of the middle 

 germ-layers and the body-cavity in Vertebrata. 



Cross section of an embryo in front of the blastopore. 



mp. Medullary plate ; ch, fundament of the chorda ; aJc, 

 outer, ik, inner germ-layer ; mi', parietal, mk', visceral 

 lamella of the middle germ-layer; d, yolk-mass; die, 

 yolk-nuclei ; dh, intestinal cavity ; Ih, body-cavity. 



collected, and the coelenteron more or less comple.ely filled with it. 

 Consequently there are formed in these cases for the production cf 

 the body-cavity tio hollow evaginations, hut solid cell-growths, in that 

 the parietal and the 

 visceral lamellce of the 

 middle germ-layer have 

 the surfaces which inAm- 

 phioxus hound the hody- 

 cavity pressed together at 

 the heginning of the de-- 

 velopment and separated 

 only at a rather late 

 stage. In order to make 

 easier the comprehen- 

 sion of the somewhat 

 dissimilar appearances 

 furnished by an inves- 

 tigation of the separate 

 classes of Vertebrates, 

 let us describe fir,st, with 

 the aid of two diagram- 

 matic figures, how, according to a series of investigations which I 

 have undertaken, the development of the middle germ-layer and 



the body-cavity would take 

 place in the case of the 

 vertebrated animals. 



One of the diagrams (fig. 

 73) represents a cross section 

 in front of the blastopore. 

 It exhibits the inner germ- 

 layer (ik) extensively thick- 

 ened on the ventral side by 

 the deposition of yolk (d), so 

 that the coelenteron is re- 

 duced to a small cavity {dh). 

 In the roof of the coelenteron 

 there lies a single layer of 

 cells (ch), the fundament of 

 the chorda, characterised by their cylindrical form. On both sides 

 of it the inner germ-layer has developed evaginations, the two 

 body-sacs (Ih), which have grown down some distance between 



Fig. 74. — Cross section of an Amphioxus embryo. 



See explanation of Fig. 70. 

 oi. Outer, life, inner, mi, middle germ-layer; ch^ 



chorda. 



