300 MR. W. K. PARKER ON THE STRUCTURE AND 
simple platform it will be easy to follow the metamorphosis of the primordial parts, 
even in the Mammal, where such changes are most of all displayed, and to compare 
and harmonize them with the lesser degrees of transformation to be seen in Fowl, Frog, 
and Fish. 
Second Stage—Embryo Pig, 1 inch long. 
Most of my illustrations of the complete embryonic skull will be made from a some- 
what more advanced stage than this; but this second stage is of great importance in 
illustrating the morphology of the facial arches and auditory sacs *. 
In this embryo chondrification has fairly set in, although the cells of the hyaline 
cartilage are still close together, quite as close as in the nidus of the vomer in the 
next stage, or the tissue in which the rostrum of the parasphenoid is developed in the 
embryo bird (* Fowl’s Skull,” Second Stage, Plate Lxxx1. fig. 7, r.st.). Ossification has 
commenced also, and can just be seen in the nidus of the vomer, maxillary, and dentary ; 
this last is the forwardest of the bony plates (Plate XXX. fig. 2,d.). Beginning at the 
snout we see that the ale nasi have chondrified, and that the retral trabecular horns 
(Plate XXX. fig. 1, a.n., ¢.tr.) have coalesced with them: the little papular prenasal 
cartilage (p.n.) is well seen in this front view; beneath this, a little further back, the 
stroma becomes dense on each side and forms the premaxillary territories, and is ready 
to ossify. In the deepest and widest part of the ethmoidal region, a vertical section of 
which (Plate XXX. fig. 2) shows the commenced ingrowing of the proper turbinal folds, 
we see now that the descending nasal roofs and the ascending trabecular crests have 
all coalesced together to form the large mesoethmoid (m.eth.). A long scoop-shaped 
territory lies immediately under the trabecular base of this septum, and this granular 
tract is undergoing endostosis; it is forming the vomer (v.). Far on each side, above a 
rudimentary tooth-pulp, is a faint trace of the maxillary (ma.). In this “schizognathous” 
stage the root of the tongue is seen at no great distance from the freely exposed vome- 
rine region, and the oral cavity (m.) has here steep sides, in the walls of which the 
primary palatal bars ( p.pg.) are seen as compressed granular plates. On each side, 
below an inferior tooth-rudiment (¢.), a large mass of nascent cartilage is seen, having a 
kidney-shaped section ; and inside this a round rod of cartilage is seen, converging towards 
its fellow of the other side as it passes forwards. If my observations had ended here, 
the thick slab of granular tissue, with its incurved edges, would have merely been noticed 
as the proper dentary territory or nidus of the mammalian mandible; it is more than 
this, as the next two stages will show: the “rod” is MEcKEL’s cartilage (m/.), the shaft of 
the first postoral arch. The dentary bone itself appears in this section, and is of a 
rich rose-colour in the preparation, one stained with carmine ; the tissue around the 
osseous deposit is becoming colourless, like MEcKEL’s rod, for the carmine scarcely tints 
the cartilage. ‘The other postoral bars are shown in this section; the ‘‘ cornu minor” 
* From a large number of exquisite sections of this stage I have only made the six illustrations here given ; 
for what the rest show is better seen in a somewhat more advanced stage, the morphological level being essen- 
tially the same. 
