DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 439 
epiblast do not separate, and therefore the layer, instead of 
becoming spread out as a disc, retains its sac-like form. The 
dorsal wall of the sac constitutes the epiblastic portion of the 
amnion, and it becomes thinned out as the cavity expands. 
There is no amnion navel, for there is no fusion of amnion 
folds ; the amnion is at first formed by a single layer of epi- 
blast, which is afterwards covered by a layer of somatic meso- 
blast, and thus in its completed state it resembles the amnion 
of other Vertebrates. The mode of its formation is, however, 
evidently secondary; it has probabiy been derived from the 
type which occurs in the rat and the mouse by the retardation 
of the separation of the margins of the epiblastic disc. Amnion 
formation in the guinea-pig is a mere variation from the 
ordinary type, and we may therefore consider that all amnia 
are produced by the folding of two germinal layers at the same 
time, either the epiblast and the hypoblast together, or the 
epiblast and the mesoblast. From folds of the latter kind the 
whole of the permanent amnion is produced in the majority of 
the Amniota; they may therefore be called true amnion folds, or, 
shortly, amnion folds. Folds of the former class are generally 
transitory ; they give way and disappear before the extension 
of the amnion folds, therefore they have been well designated 
by van Beneden pro-amniotic folds. 
In the rat and the mouse the tail amnion fold is the first to 
appear (fig. 14, Pl. XXIV). Its production is due to the growth 
of the mesoblast at the posterior end of the embryonic area, 
and it is at first quite independent of the formation of the 
ceelom. In the same manner the lateral amnion folds are 
formed as the mesoblast extends round the margins of the 
epiblastic cylinder ; and last of all the rudiment of the cephalic 
fold appears (fig. 15, Pl. XXIV). But whilst the posterior and 
lateral folds are produced by ridges of mesoblast projecting 
against the margins of the epiblastic cylinder, the cephalic 
fold consists of two portions,—one caused by the formation of 
the mesoblast, a trne amnion fold; and a second, further back 
(4 M P.), by a fold of the two primitive layers of the germ. 
It is, therefore, a pro-amnion fold, which corresponds closely 
