DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 



113 



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 no hollow evaginations, but solid cell-growths, iti that 

 the parietal and the 

 visceral lamellce of the 

 middle germ-layer have 

 the surfaces which in Am- 

 phioxus bound the body- 

 cavity pressed together at 

 the beginning 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 first, 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 

 vertebra ted animals. 



One of the diagrams (fig. 

 73) represents a cross section 

 in front of the blastopore. 



kg7 ^g^gj-,dA It exhibits the inner germ- 



\st?\ y$5l- layer (ik) extensively thick- 



^^^^^jT^rr^^\~ enec * on t ^ ie ventra * s ^ e ^7 



the deposition of yolk (d), so 

 IJjjpr that the ccelenteron is re- 



74. Cross section oi an Amphioxus embryo. duced to a Small Cavity (dh). 

 See explanation of Fig. 70. j n th( , rQO f Q f t h e coelenteron 



ale, Outer, ik, inner, mk, middle germ-layer ; ch, -, ~ 



chorda. there lies a single layer cf 



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 

 ly-sacs (lh), which have grown dow r n some distance between 



8 



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

 germ- layers and the body-cavity in Vertebrate. 



Cross section of an embryo in front of the blastopore. 



mp, Medullary plate; ch, fundament of the chorda; ale, 

 outer, ik, inner germ-layer ; mk 1 , parietal, mk 3 , visceral 

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

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



-tt 



