18 
W. H. CALDWELL. 
the early gastrula stages. In the endoderm cells little spherical 
masses of apparently the same material as the body of the cells 
themselves are frequently found. Each little mass in hardened 
embryos is separated off by a clear space. I have traced these 
bodies from their birthplace into the body cavity. They 
never possess a nucleus, and they disappear at a very early age. 
Their significance remains unknown to me unless they be 
merely an excess supply of nutriment analogous to food yolk. 
Foettinger has arrived at somewhat extraordinary results. He 
says, “ J’ai non seulement constate l’existence des premiers 
elements-mesodermiques k des stades plus jeunes que celui 
signale par Metschnikoff, mais encore je crois pouvoir reculer 
leur premiere apparition jusqu’a l’oeuf en voie de segmenta- 
tion.” The bodies referred to the mesoblast are, I believe, 
either due to the reagents used in preparing the embryos, or 
are the bodies referred to above. I have observed them fre- 
quently, but it is certain that they have nothing to do with 
the true mesoblast, whose origin I shall now describe. 
Before the lips of the blastopore meet there is no meso- 
blast (fig. 1). When the closing of the blastopore has already 
extended sufficiently far forwards to shut off a small archen- 
teric cavity, two pouchings of the endoderm occur on either 
side of the blastopore (fig. 8, ad). Each pouch is longi- 
tudinally extended in the direction of the long axis of the 
body, and is deeper towards its anterior end. The endoderm 
cells covering the region of the pouch now undergo some 
division (fig. 8), and a mass of cells is budded off on either 
side (me'). These cells as they are formed arrange them- 
selves into a sac enclosing a cavity (fig. 17, ad). These cavities, 
however, never communicate with the cavity of the gut. 
The pouch of the endoderm is soon obliterated, and the 
cells return to the size of the other endoderm cells. The 
hind part of each pouch lies about opposite to the most anterior 
point where the lips of the blastopore have closed. On 
either side of the primitive streak a few mesoblast cells are 
budded off from the cells forming the primitive streak (fig. 
18, me"). Behind the primitive groove becomes deeper, and 
