Charles L. Edwards and Clarence W. Hahn 349 
state that it arises, in part, from hypoblast budding before invagination 
and in part grows forward as a solid mass from the anterior region of 
the mesoblast sac in the middle of which the chorda becomes differen- 
tiated (Weldon, 83, Balfour, 79, Will, 95). Other investigators derive 
all the mesoblast from the sides of the chorda and from a lateral growth 
of the primitive plate after invagination has begun (Wenckebach, 91, 
Strahl, 83). There is a very close resemblance of the cross sections of 
Phrynosoma embryos in late gastrula stages, to cross sections of Lacerta 
and the Gecko figured by Will (95b, Figs. 43°, 44°, Taf. 7; 93, Figs. 59b, 
Taf. 10-Taf. 36 and of Clemmys figured by Mitsukuri, 92). Mitsukur1 
interpreted this condition as showing that the lateral mesoblast has been 
derived by a process of cell budding and an outgrowth from the edges of 
the chorda and the median margin of the hypoblast. Assuming that the 
absence of true hollow lateral sacs in Lacerta does not forbid one from 
regarding the lateral solid growth of mesoblast as having its origin in 
the primitive streak together with epiblast infolded on either side of the 
chorda anlage, one must regard the two layered mesoblast as having 
arisen in part from a posterior source and having migrated forward and 
also from the sides of the mesoblast sac as far forward as it may have 
extended. This agrees perfectly with the decrease in mesoblast as one 
passes forward as found by Mitsukuri in Chelonia (Mitsukuri, 92, Figs. 
14,15 and 16). But from this point of view, one cannot say that the 
mesoblast is derived from the hypoblast, nor is there any evidence in 
Phrynosoma that such is the case. The close connection of the hypo- 
blast and splanchnic layer ef the lateral mesoblast is a necessary conse- 
quence of the breaking away of the hypoblast and lower wall of the me- 
dian invagination, or mesoblast sac, placing the chorda in contact with 
the hypoblast anteriorly and leaving the broken edges of splanchnic 
mesoblast and hypoblast on either side to grow together later as Mit- 
sukuri describes when he says the point* moves to the pointy (92, Fig. 
16). According to the interpretation here given the somatic mesoblast 
will probably receive additions farther anterior than the splanchnic, the 
latter having a posterior source as its feeder and the former both a pos- 
terior source and possibly anteriorly from the chorda anlage. By compar- 
ing Fig. 13 with Fig. 15 we find the splanchnic mesoblast decreasing in 
lateral extent. In fact, there is practically no evidence in Phrynosoma 
that either the chorda or hypoblast contributes anything toward the ori- 
gin of the mesoblast. Will, in his descriptions of the Gecko, has already 
presented the view here set forth, but Mitsukuri, 92, states that in Che- 
lonia the hypoblast turns upward laterally to become the splanchnic layer 
of mesoblast and that the chorda contributes to the somatic mesoblast. 
