DEVELOPMENT OF THE TWO MIDDLE GERM-LAYEES. 115 



consequence of the fact that the greater collection of yolk requires 

 the space for itself. Consequently we find, in place of the body-sacs 

 exhibited in the diagram, solid masses of cells, for which it remains 

 to be established that they correspond to the sacs in position and 

 development. 



In order to see what condition would result in consequence of a 

 disappearance of the body-cavity, we will imagine that in the two 

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

 firmly pressed together. In the first diagram (fig. 73) we should 

 then have a mass several cells thick, which would be everywhere dis- 

 tinctly separated from 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 region 

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

 from the inner layer has taken place. At this point the cell-mass is 

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

 the other with the entoderm. In the second diagram (fig. 75) we 

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

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

 as to the inner germ-layer takes place. If, in addition to this, we 

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

 together from right to left, we should have in the middle of the 

 cross section a thick, many-layered cell-ma^s, which on both sides is 

 resolved into the three germ-layers, or, in other words, at the blasto- 

 pore all three germ-layers by their fusion meet together in a single- 

 mass of cells. 



By careful investigation it is, in fact, demonstrable that similar 

 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 sections through three 

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

 the blastopore, (2) through the region of the blastopore itself, and 

 (3) behind it. The agreement appears most prominent in the develop- 

 ment oj the Amphibia, among which the Tritons again furnish the 

 most instructive objects. 



When in the case of Triton the gastrulation, with the accompany- 

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

 becomes slightly elongated; the future dorsal surface (fig. 76 Z>) 

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

 stretches from the anterior to the posterior end nearly up to the 

 blastopore (u). The latter has now assumed the form of a longitu- 



