114 
ME. W. K. PARKER ON THE STRUCTURE AND 
blast,” enfolded very largely in their differentiation by involutions, tuckings-in, of the 
“ epiblast;” these are mainly on the under surface of the head, which now just projects 
free of the yelk mass. This subcranial mass of tissue becomes thickened in certain direc- 
tions, puckered into more distinct ridges and furrows, and cloven into bands or rods, the 
epiblast freely growing into every available cleft, and giving additional separateness to 
the mesoblastic structures. Hence, in the cephalic part of the embryo, the so-called 
mucous membrane, that of the mouth and fauces, is not a hypoblastic production, but is 
formed by ingrowths of the “ epiblast” down to the point where the true mucous mem- 
brane begins ; the lining of the mouth and fauces should be rather called “ inner skin ” 
than “ mucous membrane.” By means of the peculiar thickenings and puckerings of 
that part of the blastoderm which underlies the projecting head, we get an upper and a 
lower palatal region ; in the higher V ertebrata, the Crocodilia and the Mammals gene- 
rally, a third and lowest palatal floor is formed by a peculiar infolding of the supraoral 
strata. 
The uppermost and also innermost 44 palatal bands ” (‘ Gaumenleisten,’ Muller, see 
his 4 Myxinoids,’ pi. 4) are formed out of a delicate tract of 44 mesoblast ” immediately 
underlying the membranous cranium, under the rudiment of the first vesicle (Plate I. 
figs. 1 & 8, v. i, tr.) ; they are invested below by a tract of “epiblast,” the primordial 
mouth-roof, which is made and exposed by the puckerings of the 44 blastoderm.” These 
are in reality the first facial arch ; their relation to the rest of the series is seen in a 
sectional view (Plate I. fig. 8, v. i, tr.) ; the huge eyeball and the dipping-in of the tissues 
to form the mouth-cavity makes their relation to the rest of the series difficult to under- 
stand. Rathke, who regarded these bands as continuations of the 44 basilar plate ” or 
44 investing mass of the notochord,” called them the 44 rafters of the cranium,” trabeculoe 
cranii. Their morphology is much more easy to be understood in the embryo of the 
Frog ; whilst the mouth is hard to interpret in that type, and comparatively easy, as it 
appears to me, in the Salmon. A much smaller object than a rafter would have served 
better as a comparison for those rods which stand in the van of the facial phalanx, 
namely a pair of curved forceps (see Plate I. figs. 1 & 2, tr.). That these bands belong 
to the same series as the rest of the facial arches, and that they are formed out of the 
same structure of the 44 blastoderm,” is evident if we examine them in the Frog (see 
44 Frog’s Skull,” Plate nr. fig. 3, i, tr ., and Plate iv. fig. 1, i, tr.), where they are the 
largest of a series gradually decreasing in size backwards. Moreover, in the second 
stage of the Frog’s skull (Plate iv. fig. 1), when the trabecular rods have acquired the 
curve inwards above which is peculiar to the series, then they are really subocular rods, 
for they lie in the very substance of the outside of the cheek, and curve round under the 
eye, there being at that time no pterygo-palatine arch in their way. But the Salmon- 
embryo at its earliest differentiation of organs shows itself to belong to a new and higher 
dispensation ; in its earliest infancy it bears the impress of a nobler type. The blades of 
these tiny forceps are perfectly simple in form, they are not riveted anteriorly (=below) ; 
this anterior part has neither the second lobe nor the outward turn, which makes them 
