DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 



113 



d - 



die 



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 ce'l-growths, in 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 ^ ?3 _ Diagram to show ^ development of the middle 



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



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



tlgatlOIl Of the Separate ~,SJ plate; *, fundament of the chorda; a*, 



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



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



let US describe tirst, With yolk . nuc lei ; 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. 

 .dh 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- 

 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 (lh), which have grown down some distance between 



8 



-- 



Fig. 74. Cross section ol an Amphioxus embryo. 



See explanation of Fig. 70. 

 ali. Outer, ik, inner, mk, middle germ-layer; ch, 



chorda. 



