96 ARTHUR ROBINSON, M.D., AND RICHARD ASSHETON, M.A. 



We have, however, paid particular attention to this point, and have 

 come to the conclusion that the anus of Eusconi gradually diminishes 

 in size by the concrescence of the ventral part of the lateral lips, as 

 indicated in figs. 10 and 18. In these diagrams, the space lettered BR, 

 inclosed between two concentric rings, represents the blastoporic rim 

 at the time of the completion of the circular blastopore, or anus of 

 Rusconi; and the shaded space, lettered BR', represents in fig. 10 the 

 position of the blastoporic rim soon after the anus of Rusconi has 

 begun to diminish in size, and in fig. 18 a later stage, when the anus 

 of Rusconi has " contracted " so much that its diameter is now only 

 about one third of its original size. 



Figs. 7, 8, 9 are from sections taken nearly horizontally through an 

 embryo in which there was no trace of neural folds, and in which the 

 anus of Rusconi had only slightly contracted. 



Fig. 10 is a diagram of the same embryo representing the changes 

 which we conclude must have taken place. 



The blastoporic rim, round which the three layers, epiblast, mesoblast, 

 and hypoblast, are all fused, which occupied, on the completion of the 

 circular anus of Rusconi, the position indicated by the space BR 

 between the two concentric circles, had in the case of the embryo 

 examined come to occupy the position of the shaded portion lettered 

 BR' in the diagram. In the same diagram the line x represents that 

 portion of the original anus of Rusconi which has become closed up by 

 the concrescence of the ventral part of the lateral portions of the rim, 

 as stated on p. 466. 



We were unable in this specimen to find any trace of the line of 

 fusion x on the surface, and as yet there was no trace of the neural folds. 

 Accordingly, in order to be able to cut our sections as near as possible 

 in the horizontal plane as desired, we had to dissect away the opposite 

 pole of the embryo, and then by observing the shape of the yolk-plug 

 we were able to cut our sections very nearly as required. 



Fig. 7 has been drawn from a section (which is not quite horizontal) 

 taken through the centre of that portion of the blastopore which was 

 still open. A.t the lips of the blastopore the three layers are seen to be 

 fused. 



The line running from ZZ ends at the furthest point from the 

 blastoporic lip to which the fusion of epiblast (EP) and mesoblast {ME) 



