!l.\ RLOPMRTT "l Till PWO MII-IH \YERS. 



113 



collet-led, and the c.rlenteron more or less completely title. 1 with it. 

 Consequently there are formed in these cases for the pn-liir ion of 



the liodv cavity no hollmr i-i'iuji nut imts, but Sof> 'l't 



tin 1 {xtr'ntnl ninl the 



'i I In in I'll 'if of tin' 

 in i'l <ll<' i/fi'iii -In >/>'/' /in re, 

 - v which in Am 

 /i/iw.i-nx hi m ml the />i>< ft/- 

 i'ii rift/ jirewi/ together at 

 thf l < i't nning of the de- 



neiit ini'l xr / >(t rated 

 "it/// at a rather Intu, 

 xttnje. In order to make 

 easier the comprehen- 

 sion of the somewhat 

 dissimilar appearances 



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. 



iullarv plute ; r/<, fund uncut <>f tin- clmrilii nk, 

 outer, ilc, inner germ-la \ inceral 



lamella of tin- miiMlr ^t-riii 

 vclk-iuiok-i ; <<h, iiite-tiiial .-a\i' 



-tt 



furnislit d by an inves- 

 tigation of the separate 

 rlasse- of Vertebi 

 let us describe lirst , with 

 the aid of two diagram- 

 matic figures, how, according to a series of investlgatioitB which I 

 have undertaken, the development of the middle germ-layrr and 



the body -cavity would take 

 place in the case of the 

 vertebrated anin.ai>. 



One of the diagrams (fig. 

 73) represents a CTO8B stKJtion 



in front of the blastopore. 



It exhil>its the inner germ- 

 layer (//,') e.\tensi\ely tllick- 



ened on the ventral side by 



the .It position of yolk i 



that tlie Melenteron is re- 

 duced to a small ca\it\ 



III the root of the C.rlei 



there lies a single lay. 



cells (r/ t ). the fundament of 



the chordfl, characterised by their cylindrical t'mm. On both si.l.-s 

 't it the inner germ-la \cr has develoj>ed exaginat ions, the twt> 

 hoily-sacs (//,). which have grown down some distance betv. 



8 



Fig. 74. Cross section of an Amphioxus embryo. 



See explanation of Fig. 70. 

 <. outer, ile, inner, ml; middle gerra-laver ; </<, 



<l;i. 



