1885.] 



and Blastopore of the Lamprey. 



245 



small cells pass gradually into the larger ones at the sides of the seg- 

 mentation cavity. All the cells of the blastosphere are crowded with 

 yolk spherules, which are, however, very much smaller in the upper 

 cells, where active division is going on, than in the more inert lower 

 cells. The latter may be conveniently termed the yolk cells. The 

 segmentation cavity is considerably larger than that of the frog's 

 ovum at a similar period. The next stage in the development of the 

 ovum is accompanied by the thinning out of the upper layer of cells, 

 until the roof of the segmentation cavity finally consists of a single 

 layer of cells. On this point my observations confirm those of Calberla, 

 and are opposed to those of Max Schultze, who found a many-layered 

 roof covering the segmentation cavity just before invagination. The 

 layer is composed of epiblastic cells, and in this respect resembles the 

 many-layered roof of the segmentation cavity in the frog's ovum of 

 the same stage. The lower cells of the many-layered roof seem to pass 

 round to the sides and floor of the segmentation cavity, so that about 

 the fiftieth hour after fertilisation the upper half of the egg consists 

 of a hemispherical segmentation cavity, roofed in by a single layer of 

 cells, the lower half being solid and composed of yolk cells. Viewed 

 as an opaque object, the upper half is of a whiter colour than the 

 lower. 



The invagination to form the mesenteron takes place in the region 

 where the single layer of epiblast cells passes into the yolk cells. In my 

 eggs the first trace of this invagination appeared about 130 hours after 

 artificial fertilisation. The mouth of the invagination or blastopore 

 is at first a wide arched slit, which subsequently narrows to a round 

 hole. From the first sign of invagination, a cavity, the mesenteron, is 

 present ; in this the Lamprey resembles Amphioxus, but differs from 

 the Frog, where the mesenteron is formed as a slit some time after the 

 invagination has begun. It differs from Amphioxus, however, in the 

 fact that the invagination is not symmetrical, being like the segmen- 

 tation cavity, pushed dorsally by the accumulation of yolk cells at the 

 lower pole. The upper layer of invaginated cells retain the character 

 of the epiblast cells, the lower are larger, and have the characters of 

 the yolk cells. The former lie close against the inner surface of the 

 epiblast cells in the dorsal median line, here again differing from the 

 Frog, where a mass of cells, which subsequently form mesoderm, lies 

 dorsally to the invaginated cells and between them and epidermis. 

 There is thus no mesoblast present at the dorsal rim of the blasto- 

 pore, such as is found in frog's egg. During these processes the 

 epiblastic cells have gradually enclosed the yolk cells ; this appears to 

 take place by the conversion of the outer yolk cells into epiblast 

 cells, and takes place latest in the region of the blastopore. The 

 mesenteron continues to deepen, and as it increases in size the seg- 

 mentation cavity diminishes, and is finally obliterated. The roof and 



