DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 



113 



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

 Consequently there are formed in these cases for the production of 

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

 the parietal and the 

 visceral lamellce of the 

 middle germ-layer have 

 the surfaces which inAm- 

 phioxus bound the body- 

 cavity pressed together at 

 the beginning of the de- 

 velopment and separated 

 onli/ at a, rather late 

 stage. In order to make 

 easier the comprehen- 

 sion of the somewhat 

 dissimilar appearances 

 furnished b an inves- 



Ih 

 dh 



,1 - 



die 



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. 



tlgation of the separate ^ Medullary plate; ch, fundament of the chorda; ak, 



classes of Vertebrates, outer, ik, inner gei-m-layer ; mk 1 , parietal, rnk", visceral 



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



5t, Wltn 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 of an Amphioxus embryo. duced to a Small cavity (dh). 



In the roof of the co3lenteron 

 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 



See explanation of Fig. 70. 

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

 chorda. 





