DEVELOPMENT OF THE TWO MIDDLE GERM- LAYERS. 



113 



collected, and the crelenteron 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, iti that 

 the parietal and the 

 visceral lamellae 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 p ^ ?3 _ Diagram to show ^ development of the middle 



furnished by an inves- germ- layers and the body-cavity in Vertebrate. 



. . , . < , i Cross section of an embryo in front of the blastopore. 



tlgatlOn Of the Separate ^" 8 edullary plate; J fundament of the chorda; ale, 



classes of Vertebrates, 'outer, ik, inner germ-layer ; mk\ parietal, ml?, visceral 



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



let US describe first, With yolk-nuclei ; dh, intestinal cavity ; Ih, body-cavity. 



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 crelenteron is re- 

 Fig. 74. Cross section oi an Amphioxus embryo. duced to a small cavity (dh). 

 See explanation of Fig. 70. J n t ^ e roo f o f the CO3lenteron 



ak, Outer, ik, inner, mk, middle germ-layer; ch, i i u 



chorda. 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 



8 



srlfir a* 



