432 ARTHUR ROBINSON. 
the posterior end of the embryonic area (45). According to 
Strahl and Carius, a portion of the intra-embryonic ccelom of 
the guinea-pig and rabbit is developed separately, and fuses 
secondarily with the general body-cavity (50 a). 
According to the accounts given by Professor Fraser and 
Professor Selenka, the ccelom in the rat and the mouse is 
formed much in the same way as in the guinea-pig. These 
observers, however, differ from each other upon some details. 
Professor Fraser, in a diagrammatic representation of a longi- 
tudinal section of a rat’s ovum at nine days seventeen hours, 
figures the distal trophoblast as only partially covered by the 
mesoblast, which is extending as a continuous layer. Pro- 
fessor Selenka considers that this is incorrect, and my obser- 
vations confirm his conclusions, but they do not support his 
assertion that a portion of the wall of the extra-embryonic 
coelom of the mouse is formed by migratory mesoblastic cells. 
Duval (9) agrees neither with Fraser nor Selenka, and 
according to his account the formation of the colom in the 
mouse is very similar to its formation in the sheep. It com- 
mences in the extra-embryonic region behind the embryo as a 
series of spaces which gradually fuse into a continuous cavity. 
My specimens do not confirm Duval’s account, but the 
appearances they present are more in accord with his descrip- 
tions than with that given by Selenka, for they show that the 
cavity appears, as in most Vertebrates, within the mesoblast, 
and not, as in the guinea-pig, between the trophoblast and 
epiblast. 
Tn the mouse ovum during the latter part of the eighth day, 
and at a corresponding period in the rat’s ovum, a knob-like 
mass of mesoblast is projected back wards from the posterior 
end of the primitive streak (fig. 14, Pl. XXIV). The growth of 
this mass pushes the posterior end of the epiblast towards the 
anterior end, and thus produces the first rudimentary amnion 
fold(A FC). Selenka calls this mass the ‘“‘ Allantoisknopse ” 
(44, 45), and states that the migratory cells which enclose the 
ccelom are budded off from its surface. It is not the allantois, 
for that organ does not appear until the latter part of the 
