336 Some Phases of the Gastrulation of the Horned Toad 
embryonic area proves that some other agent maintains the elevated po- 
sition of the thickened epiblast. 
As in other reptiles the upper and lower layer are connected by a mass 
of mesoblast at the posterior edge of the embryonic area. Here by the 
invagination of the primitive plate of Will is formed the mesoblast sac 
(Mesodermsackchen of O. Hertwig, 03, p. 828). That all of the meso- 
blast in Figs. 2 and 3 was derived from the proliferation and invagination 
of the region of the primitive plate and blastopore, can not be positively 
affirmed, yet it seems highly probable. The mesoblast at the anterior 
margin in Fig. 3 is in direct connection with the mesoblast under the 
primitive plate. 
The mesoblast cell masses are continuous with the epiblast of the pos- 
terior and lateral lips of the blastopore which leaves little doubt as to 
their origin. From these two sources there is a lateral and a forward 
extension of the mesoblast cell masses. From the floor of the mesoblast 
sac one mass extends forward in the middle region (Fig. 2, ms’bl.) 
It is continuous laterally with the lateral extensions, the floor of the 
mesoblast sac contributing to the lower portion of these also. The in- 
turned epiblast, or lateral roof of the mesoblast sac is con- 
tinued laterally and forward by a mass of mesoblast (Fig. 3, ms’bl.™) 
having a high position. Laterally this mesoblast bends down and back 
(Fig. 2, ms’bl.“") to unite with the mesoblast from the fioor of the meso- 
blast sac (Fig. 2, ms’bl.#). It thus forms a pouch on either side which 
is continuous posteriorly with the mesoblast sac in the median line. The 
floor of the mesoblast sac is continued anteriorly for a short distance 
under the chorda anlage into a sheet of mesoblast. In a middle position 
it spreads laterally over the Jateral pouch above mentioned, while farther 
back, as the floor of the mesoblast sac, it is continuous with the ventral 
wall of the lateral pouches. 
Under the region designated primitive plate (Fig. 2, la. pr.) the meso- 
blast is in the form of a network, and, while it seems to be spreading 
from these centers of origin, it is not being encroached upon by new 
additions, or proliferation, inter se. Mesoblast of this nature can be 
traced from the hollow pouches extending laterally on either side of the 
primitive streak, to the anterior margin of the embryonic area where it 
fills in the space between the epiblast and hypoblast (Fig. 3 ms’bl.i), 
According to Will a similar forward growth of mesoblast on each side 
of the chorda takes place in Lacerta agilis, L. muralis and a like condi- 
tion exists in the turtle (Mitsukuri, 94), snake (Will, 98) and in Spheno- 
don (Dendy, 99). 
The mesoblast sac becomes a canal reaching far into the mass of meso- 
