TO THE EMBRYOLOGY OF AMPHIBIA. 47 



point of its first appearance in the dorsal median line, along the 

 lower edge of the equatorial zone, until the two limbs of it meet 

 each other and complete the circle in the ventral median line. 

 Before the lip becomes established, the component zonal cells are 

 no doubt differentiated into the permanent epiblast cells, and by 

 this means the epiblast grows down toward the lower pole. As 

 the downward growth of the epiblast is thus taking place before 

 the lip is definitely established, it seems to me that the individual 

 or specific variations in the exact spot at which the dorsal blasto- 

 pore lip first appears, are not of so much importance as some 

 writers believe. 



The establishment of the lip itself, we must regard as due 

 to invagination. While this goes in deep at the dorsal portion, 

 it becomes at all other portions of its circumference evaginated 

 again to add to its downward growth, as explained below. 



The growth downward of the blastopore lip, after once it is 

 established, is, I believe, nothing but the continuation of the cell 

 mutiplication and the differentiation of these cells in situ. Al- 

 though I have no direct proof of it, yet I have no doubt that the 

 most active seat of cell-multiplication is at the edge or just inside 

 the edge of the lip. Cells multiplied accummulate inside the lip 

 as a sort of invagination and become soon evaginated to add to 

 the downward growth of the lip. This takes place all around the 

 circumference of the lip except at the dorsal median line where the 

 underlying archenteric cavity has to be deepened and enlarged. 

 The formation of the blastopore lips and their growth are therefore 

 not produced by either invagination or cell-multiplication alone, 

 but conjointly by these two processes. 



4). As to the manner in which the segmentation-cavity 

 diminishes in size, most previous writers have stated that this 



