FORMATION AND FATE OF THE PRIMITIVE STREAK. 467 



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 indi- 

 cated by the space BR between the two concentric circles, had 

 in the case of the embryo examined come to occupy the posi- 

 tion of the shaded portion lettered BR in the diagram. In 

 the same diagram the line or 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 a^ 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. At 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 meso- 

 blast (ME) extends on one side, and the line from ZZ ends at 

 the furthest point from the blastoporic lip to which the fusion 

 of those layers extends on the other side. It will be noticed 

 that on one side the fusion extends further than on the other. 

 No doubt this is chiefly owing to the section being not quite 

 horizontal, the side of the ZZ' being the more ventral of the 



