Il6 EARLY STAGES IN THE 



spheres are much larger than where the yolk-granules are more 

 sparsely scattered. 



The Frog is the vertebrate whose development comes nearest 

 to that of Amphioxus, as far as the points we are at present 

 considering are concerned. But it will perhaps facilitate the 

 understanding of their relations shortly to explain the dia- 

 grammatic sections which I have given of an animal supposed 

 to be intermediate in its development between the Frog and 

 Amphioxus. Plate 5, fig. B I, represents a longitudinal section 

 of this hypothetical egg at the close of segmentation. The 

 lower pole, coloured yellow, represents the part containing more 

 yolk material, and the upper pole, coloured blue, that with 

 less yolk. Owing to the presence of this yolk the lower pole 

 even at the close of segmentation is composed of cells of a 

 different character to those of the upper pole. In this respect 

 this egg can already be distinguished from that of Amphioxus, 

 in which no such difference between the two poles is apparent at 

 the corresponding period (Plate 5, fig. A l). 



The segmentation cavity in this ovum is not quite so large 

 proportionately as in Amphioxus, and the encroachment upon 

 it is due to the larger bulk of the lower pole of the egg. In 

 fig. B II the involution of the lower pole has already com- 

 menced ; this involution is (l) not quite symmetrical, and (2) 

 on the ventral side (the left side) the epiblast cells forming the 

 upper part of the egg are growing round the cells of the lower 

 pole of the egg or lower layer cells. Both of these peculiarities 

 are founded upon what happens in the Frog and the Selachian, 

 but it is to be noticed that the change from the lower layer cells 

 being involuted towards the epiblast cells, to the epiblast cells 

 growing round the lower layer cells, is a necessary consequence 

 of the increased bulk of the latter. 



In this involution not only are the cells of the lower pole 

 pushed on, but also some of those of the upper or yellow por- 

 tion ; so that in this as in all other cases the true distinction 

 between the epiblast and hypoblast does not appear till the 

 involution to form the latter is completed. In the next stage, 

 B in, the involution has become nearly completed and the 

 opening to the exterior or blastopore quite constricted. 



The segmentation cavity has been entirely obliterated, as 



