, 
348 Some Phases of the Gastrulation of the Horned Toad 
epiblast and hypoblast of the horned toad than in either of the forms just 
mentioned. This may account for the greater development of the lateral 
mesoblastic pouches of the horned toad. No figures of cross sections are 
given by either Wenckebach, or Will, that represent such a lateral exten- 
sion of the mesoblast sac, yet it seems probable that they may exist. 
There is in the blastoderm of the Gecko a condition which corroborates 
the interpretation we have given above of the gastrulation in Phryno- 
soma, namely that there are two paired, and one median, hollow pouches 
derived from the sides and front of the mesoblast sac respectively. In 
Will’s stage III (Fig. 48 Pl. 7) the nresoblast sae is very shallow, the 
head process is in evidence and presumably some lateral spreading of 
mesoblast. In stage IV (Fig 55, Pl. 9) the head process, through the 
extension of the mesoblast sac has become a long hollow blind sac, flat- 
tened considerably between epiblast and hypoblast. By the disintegra- 
tion of the lower wall of the sac and of the underlying hypoblast (Fig. 
57, Pl. 9) the mesoblast sac is placed in communication with the seg- 
mentation cavity. It is obvious that this median sac of the Gecko gas- 
trula is homologous with the median sae of the Phrynosoma gastrula 
and that the condition shown in Figs. 2 and 3 is derived in the same 
manner as that in Will’s Fig. 57, Pl. 9. But Will makes no mention of 
lateral pouches which are doubtless more conspicuous in Phrynosoma 
because as we have already pointed out, in the latter, there is no dorso- 
ventral compression. Will’s explanation of the continuity of the chorda 
with the hypoblast in the anterior median region of the blastoderm is in 
full accord with conditions found in late gastrula stages of Phrynosoma. 
Will describes a similar median pouch in Lacerta viridis (Will, 95b, Fig. 
36. RataGi)ie 
In the early development of some lizards one does find a slight eleva- 
tion of the posterior margin of the blastoderm (Will, g5a, Fig 36, Taf. 
6, L. viridis; Will, 93), but in no sense comparable to the conditions in 
Phrynosoma. In other cases there is no suggestion of it (Weldon, 83, 
Wenckebach, g1, Strahl, 82). Conditions in the formation of the em- 
bryonic area and mesoblastic pouch in Lacerta are substantially the same 
as in the Gecko (Wenckebach, 91). There are some minor differences in 
the development of the chorda. The unpaired median invagination even 
at the very beginning forms a compressed triangular shaped cavity in the 
lizards, while it is much less compressed at first in Phrynosoma, becom- 
ing more and more compressed later. As in the Gecko, the genus Lacerta 
does not develop mesoblast by prominent hollow lateral pouches from 
the antero-lateral sides of the mesoblast sac. In fact, there is some con- 
fusion as to the exact origin of the mesoblast. Some authors (Will, 95b) 
