DBVBLOPMXKT Of 'MM: TWO MII.M.I: <.KKM i. \YER8. 119 



appear in stronger relief. The of separation luced 



lv the curving of the I-/,,,,-,/,!/ plate, and its oonvornon into the 



clmrilnl '//""''' li'- r - "'' ' '"'> Inasmuch as it i- OOntl&llO 



3 with tin- parietal lamella of tin- middle gi'nn-la\v there 



arise in the root' of the ccelenteron the two small chorda] told-, which 



elldo-e hrt \\eell tlu-lM the chorda 1 groove. I Is free margins alillt 



direct Iv upon the folded edir<'. whore the vix-n-al lamella of the 

 middle LTerm-laver I//// 1 -) hends art mud into the entoderm (ik) to 

 produce the cu'lentenc fold. 



In the next following Mage (fig. 79 B) the thickened medulla! -y 

 plate, con-i-ting of long cylindrical cells, lieconie> distinctly marked 

 otl' from the now still >maller cuhical elements of the ectoderm. 

 Meanwhile the middle germ-layer Kevins to detach itself from it- 

 previous connections in the \icinityofthe place of e\ agination ; the 

 parietal lamella becomes separated from the fundament of the 

 chorda, the visceral lamella from the entoderm, and thereupon their 

 dei a died edges become fused to each other. By means of this pro- 

 cess the fundament of the body-sac, or of the middle germ-layer, 

 becomes closed on all sides, and is separated from the other 

 LTi-rm la\er>. At the same time the entoderm (//) and the funda 

 ment of the chorda (ch] have come into contact alonr their free 

 margins, so that the chorda appears like a thickening of the ento- 

 derm. and for a time -hare- in !,.undinir the inte-tina! cavity on the 

 dorsal -ide. This is changed by a second process of detachment. 



The fundament of the chorda, now converted into a solid rod, 

 is gradually excluded from participation in lining the intestine 

 (n'g. 79 C), by the fact that the halves of the entoderm (ik), composed 

 of largo yolk-cells, grow toward each other underneath it, and fuse 

 in a median raphe. 



The closure of the per inn in- nt inf^ffni' an the dorsal ride, the con- 

 Striding of of the two body-sacs from /// 



nr'xjln f the chorda dorsalis are therefore in Ainjihft>fn, as in Am 

 oxus, processes ivhich are most intimately related with one aii< 

 Here, too. 'ing offoj the part* / begins at tfie head-end 



of the embryo, and advances slowly toward fh>- posterior end, where 

 there exists for a Ion i N '6 of growth . Ay means of which the 



',< the length of the //// /.v effected. Soon after this, the 

 moment arrive- when in the embryos of Triton the body-cavity 

 becomes visible. For after the detachment of the organs previously 

 mentioned i- completed, the two middle germ-layers at the head end 

 of the body, and on both sides of the chorda, separate from - 



