DEVELOPMENT OF PETROMYZON FLUVIATILIS. 177 



large and occupies the whole of the upper hemisphere, whilst 

 the lower hemisphere is solid and consists of larger cells, which 

 we may speak of as yolk-cells. The most external layer of 

 these consists of rather columnar cells. These latter cells 

 soon become smaller than the inner yolk-cells, and about the 

 time of invagination the whole egg is enclosed by a layer of 

 small columnar cells, the epiblast. This is brought about by 

 the conversion of the outermost row of yolk-cells into small 

 columnar cells. As Balfour has shown, this takes place 

 latest in the region of the blastopore. 



The invagination which forms the mesenteron commences 

 about 130 hours after fertilisation ; it commences at one side 

 of the equator of the egg, in the region where the single layer 

 of epiblast cells passes into the yolk-cells (fig. 9). The invagi- 

 nation at first has a wide-arched slit-like opening, but this 

 soon narrows into a small circular pore (fig. 4). The segmen- 

 tation cavity is gradually obliterated by the invagiaated ceils. 

 These from the first enclose a cavity, the mesenteron. In this 

 respect the formation of the gastrula is like that of Amphioxus, 

 and differs from that of the Amphibia, where the mesenteron 

 appears later as a splitting underneath the invaginated cells. 

 The presence of a large amount of food-yolk causes the invagi- 

 nated cells to be pushed dorsalwards. The mesenteron 

 extends as a tubular cavity about two thirds round the 

 embryo. Its dorsal wall is composed of columnar cells resem- 

 bling those of the general epiblast ; the cells forming the floor 

 have the same characters as the yolk-cells (fig. 12). The 

 dorsal side of the mesenteron lies in immediate contact with 

 the under surface of the epiblast throughout its entire length. 

 In this respect again the Lamprey differs from the Frog, where 

 the invaginated hypoblast cuts off a mass of cells on its dorsal 

 side, which subsequently forms the mesoblast. 



The mesoblast now appears by the differentiation of two 

 bands of these yolk-cells, which lie in the angles formed by 

 the mesenteron and the epiblast (fig. 12). This differentiation 

 commences in front and is continued backward. The two 

 bands of mesoblast are separated dorsally by the juxtaposition 



