510 The American Naturalist. [June, 
of the primitive streak, Pr. In the lamprey there is at 
this stage no such accumulation of cells; according to Shipley 
the ectoderm consists of a single layer of cells and the arch- 
enteron is bounded on its dorsal side by a single layer of cells 
also, between which and the overlying blastoderm there are 
no cells; the gathering of cells, corresponding to the primitive 
streak, does not arise until later. The cavity becomes that of the 
anterior end of the archenteron ; it is sometimes designated as the 
blastoporic invagination. The archenteron, in the same measure 
as the blastoderm spreads 
over the yolk, grows at 
its hinder end away from 
the segmentation cavity, 
Fig 6, just as in elasmo- 
branchs. A stage is 
soon reached in which 
nearly the entire length 
of the archenteron is 
formed, and nearly the 
whole yolk is covered. 
Fic. 7.—Longitudinal section of the ovum of a kheng ip stil abi naoRorg 
tu n after the formation of the entodermic cavity. which leads into the 
Ec, ectoderm; mes, mesoderm; Ent,entoderm; B/ : i 
blastopore; 47, diverticulum of the archenteron; Vi, Cavity, and which has 
Sp aeeti moved gradually back- 
wards from its original position. Behind the blastopore lies 
the uncovered yolk, which in the frog’s ovum is very conspic- 
uous, because its whitish color contrasts with the dark color 
of the heavily pigmented ectoderm around it; this area of ex- 
posed yolk is the so-called anus of Rusconi. When the 
archenteron has completed its full length, the following dis- 
position of the parts is found, Fig. 7. The archenteron is 
bounded below by the large mass of yolk cells, vê, and above 
by the epithelium, ext, of the entoderm; its posterior end 
curves up to open at the blastopore, B/, passing through a 
mass of cells, which constitutes the hind end of the primitive 
Streak ; this portion of the archenteron is sometimes called the 
blastoporic canal. There is further a short prolongation, A/, of 

