396 ARTHUR ROBINSON. 
bryonic area produces the head amnion fold (AFH, fig. 15). 
The continued growth of the mesoblast and the appearance 
and extension of the body-cavity (figs. 15 4d and 15 B) are 
associated with the completion of the amnion folds and the 
separation of the cavity of the trophoblast from that of the 
epiblast, the latter cavity now becoming the amniotic sac; 
at the same time the distal margins of the trophoblast are 
fused, and thus the trophoblastic cylinder acquires a distal 
wall (DWT, fig. 15 B), which is gradually invaginated into 
the cavity of the trophoblast coincidently with the extension 
of the celom, and finally the cavity of the trophoblast is 
obliterated (fig. 19, Pl. XX VII). 
Whilst the coelom is extending a solid mass of mesoblast is 
projected into it from the posterior end of the embryonic area. 
This is the rudiment of the allantois (fig. 19, Pl. XX VII). 
According to Fraser’s account (13) the mesoblast is pro- 
duced by the budding off of cells from the epiblast. at the 
hinder end of the embryonic area, over which portion of the 
ovum the middle layer spreads rapidly in the form of two 
lateral plates, which are not continuous across the middle line. 
In addition to this embryonic portion the mesoblast also 
spreads in another direction, splitting at the anterior and pos- 
terior ends of the embryonic area; one part of it passes over 
the amniotic portion of the neuramniotic cavity; the other 
passes internal to the hypoblast over the free surface of the 
epiblastic wall of the false amnion cavity. 
Selenka also ascribes to the mesoderm a purely epiblastic 
origin, for he describes it as formed by “eine zellenwucher- 
ung des Ektoderms in Gestalt einer Platt auf, welche sich in 
den Spaltraum zwischen Ektoderm und Entoderm hinein- 
drangt. Gegen den Trager hin wuchert gleichzeitig aus dem 
ausseren hinteren Umschlagsrande der Primitivrinne die 
Allantois als Knopse hervor” (44, p. 17). 
If these opinions stood alone, they would go far to support 
Kolliker’s (26) description of the mesoderm as a secondary 
formation from the epiblast; but Duval is convinced ‘ que 
Vétude du blastoderme de la souris, malgré ses dispositions si 
