620 
MR. W. K. PARKER ON THE STRUCTURE AND 
Here, in Lacerta, I have been able to verify a large number of his observations on 
the development of the embryo in the Sharks and Skates; and such things as are 
fundamental to my own special work I must introduce here. 
The embryo has now well nigh completed the number of its somatomes; there are a 
few more than in the last stage. The “ metasomatomes” have now the work of develop¬ 
ing the vertebrae. 
These embryos are often very twisted and unsymmetrical, the meso-cephalic flexure 
is complete, so that the mid brain (Plate 38, figs. 1, 2, C 2 ) bulges forwards, in a line 
with the axis of the embryo. 
The hind brain (C 3 ) bulges but little, and its boundary is indefinite behind, where 
it runs into the “ myelon the fore brain (C 1 ) is half covered by the budding, sym¬ 
metrical hemispheres (C 1 "), and these again are overlapped by the rudiments of the 
nasal capsules ( ol .). 
The eyeball (e.) has almost doubled its relative size, but its folds are still separate 
below; moreover, it has made its orbital “ nest ” neatly, by its own swelling bulk and 
pressure. 
The “strabismus” of the eyes is nearly equal to that figured by me in the embryo 
Salmon, that obliquity, however, was artificial (see “ On the Skull of the Salmon,” 
Phil. Trans., 1873, Plate 1). 
The ear-sac (ciu.) is now an elegant “ lagena,” with a short neck; the “clefts” (cl) 
are enclosed below ; the pericardial pouch is not yet covered, but the abdominal walls 
are fast closing in ; letting out, however, the large allantois (all.), which is lining all 
the egg-wall within. 
The fore limb ( pt.l .) is becoming knuckled, so that the three main regions—arm, fore¬ 
arm, and hand—can be seen ; the hind limb (pv.l.) is still a mere oval bud. 
When the twisted head is tilted so as to show the base (Plate 37, fig. 7), several 
important things are better seen (compare that figure with fig. 1 in Plate 38). 
In this figure, which is seen at once to have the same liippopotamoid form as the 
early embryo of the Pig (“Pig’s Skull,” Plate 28), we see that the angles of the huge 
oral involution are serially homologous with the gaping spaces behind the mouth 
(cl, 1-4). 
Also it would seem, to any unprejudiced observer, that the fold above the mouth, on 
which the eyeball rests, is the serial homologue of the folds behind the mouth. 
That is to say, the palatine fold (“superior maxillary rudiment,” “ maxillo-palatine 
fold ”) appears to be the morphological equivalent of the folds next following, in 
which are developed the mandible, hyoid arch, and branchial arches. 
I do not think that this is invalidated by the fact that the “ hypoblast ” ceases 
inside the first post-oral fold, for the hypoblast ceases at or near the anal aperture, yet 
this does not so affect the vertebrse as to stop the growth of (at least rudimentary) 
haemal arches in the caudal region. 
The first prce-oral visceral fold (right and left moiety of “ naso-frontal process”) is not 
