DEVELOPMENT OF THE TWO MIDDLE GERM-LAYEUS. 



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 tlie chorda ; ak, 

 outer, ik, inner germ- layer ; mk\ parietal, mk", visceral 

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

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



collected, and the ccelenteron more or less completely filled with it. 

 Consequently there are formed in these cases for the production cf 

 the body-cavity no hollow evaginations, but solid cell-growths, in 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 

 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 ccelenteron 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 chordn, 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 



8 



Fig. 74. Cross section of an Amphioxus embryo. 



See explanation of Fig. 70. 

 alt, Outer, ik, inner, mk, middle germ-layer ; ch, 



chorda. 



