MKM 01 mi. rwo vii.m.i: .I.I:M \.\\ 1 1 ~> 



ili fact that the greater 



the -pace tor iiM-lf. ( 'on>e(|uently w find, /'// /i////v <>/' /sacs 



!!, -a in, ftuff'f masses of fliirf, it ,-> mains 



ffitnlitif!/,*'!/ t/int (! // <-" i- respond to the sacs in and 



In order to >.-e what condit ion would re>ult in consequence of a 

 disappearance of the body ca\ it y, we will imagine that in the two 

 diagrams the parietal and the visceral layers of the body-sacs are 

 lirmly pre>.M-d together. In the first diagram (fig. 73) we should 

 then have a mass several cells thick, which would be every wl MM 

 tinctly separated t'roin the two germ- layers in between which it had 

 grown with the exception of the place indicated by a star, which 

 marks the entrance to the body -sac ; this is the important r--_ 

 whence the evagination or the outgrowth of the middle germ-i 

 from the inner layer ha^ taken place. At this point the cell-m.i 

 continuous, on the one side with the fundament of the chorda, on 

 the other with the entoderm. In the second diagram di_ 

 should likewise see the thick cell-mass everywhere isolated, except in 

 the vicinity of the blastopore, where a transition to the outer as 

 as to the inner irerm-lnyer takes place. If, in addition to this, we 

 should imagine that the two lips of the blastopore were here pressed 

 iier from ri^ht to left, we .-hould have in the middle of tlie 

 ion a thick, many layered cell-mass, which on both Bid 



resolved into the three germ-layers, Sfc; in other words. <// (he blasto- 

 pore all three f/tv///-A///<'/-x A// t/i?ir fusion nt>>>i {</>/// /'// .% 



1 nf Ct'lls. 



I'.y careful investigation it is, in fact, demonstrable that >imilar 

 conditions to those which we have produced by changes in the 

 diagrams are found in the investigation of the several classes of 

 Vertebrates. For this purpose we must make >ectioii> through three 

 ditiereiit regions of the embryo : (1) through the region in front of 

 the Ma>topore, (2) through the region of the bln>tpore itself, and 

 ( ."> i behind it. 77- /*/ nppears m< <n th>~ <! > 



' J the Ainjthittiit. aiming which the Tri; D furni>h the 



nnt insti-uctive object >. 



When in the case of Triton the ira^trulat ion. witli the accompany- 

 ing obliteration of the cleavage-cavity, is fully completed, the embryo 

 Mies slightly elongated; the future dorsal surface (fig. 76 D) 

 becomes flattened, and gives rise to a shallow furrow (r), which 

 Mi-etches from the anterior to the posterior end nearly up to the 

 blu*t opore (u). The latter has now assumed the form of a longitu- 



