144 
ME. W. K. PAEKEE ON THE STETTCTUEE AND 
second, and their section is most perfectly ovoidal. In the rest the shape in section is 
round within and flattish on the outside; this arises from the fact that dehiscence 
commences on the inner side several days before the clefts appear in the cutaneous 
system; yet thus early the cheeks are becoming forrowed (figs. 6 & 7) between the 
arches. 
The incurved lower end of the arches stops short some distance from the mid line, 
leaving a space for the azygous pieces, which, however, do not appear for several days to 
come. The oblong tract of tissue between the arches below (tig. 7), which has a rounded 
free margin anteriorly, and which has been partly cut away in the section behind, is the 
basihyal and basibranchial region ; the free-ending anterior part becomes the tongue ( tg .). 
The tissue of which the rods are composed is cellular, very solid, and is rapidly passing 
into hyaline cartilage ; a very evident concentric line indicates the differentiation of the 
perichondrium ; they are imbedded between the skin and mucous membrane in a very 
thin layer of delicate gelatinous tissue. A transversely vertical section (fig. 8) displays 
the manner in which the first pair, or “ trabeculae,” underprop the membranous cranium ; 
the section is through the “ first cerebral vesicle ” and the rudimentary eyeball ( e .). Here 
these rods seem, as seen from behind, to come into contact more nearly than they do in 
reality; this is caused by their being seen through the thick mucous membrane; the 
other sections (figs. 5-7) correct this. Immediately below the clubbed ends of these 
rods (fig. 8) is the upper lip ; this contains a trace of solidifying cartilage of the upper 
labial, seen better in figs. 3 & 4 (u.L). Below the mouth (pi.) there is another and more 
vertical patch of young cartilage (figs. 3, 4, and 8, l.l .) ; this becomes the lower pair of 
labials ; the first poststomals are seen from behind, and partly cut away in fig. 8 : the 
great density and thickness of the cutaneous investment, and the tracts filled by gelati- 
nous tissue, are shown in this figure, as also the manner in which the membranous 
cranium rests upon the palatal portion of the oral mucous membrane. 
In the horizontal section of a hatched embryo 3 lines long (fig. 9) we have a transition 
towards the second stage. 
The decreased “ mesocephalic flexure ” has carried the trabecular rods into a more 
horizontal position, so that the section of them is very oblique and not directly across as 
in fig. 7. 
The next pair have freed themselves below from the better-formed rudiment of the 
tongue (tg.), and the free clubbed ends of these rudiments of the mandibular arch are 
now preparing for transverse fission ; thus “ Meckel’s cartilage ” will soon be differen- 
tiated. The next or hyoid arch is now seen to belong to the rudimentary tongue ; the 
gill-buds are seen enlarging outside the first and second branchials ; the fourth branchial 
is not yet distinct. In both the lateral views (figs. 2 & 3) the skin has been removed 
from over the auditory sac (cm.): it was perfect, however ; but the sac itself, already with 
very solid walls, is open on the outer side ; this opening is large, oval, and turned down- 
wards and forwards ; it looks at this stage as though the whole structure had been formed 
as an involution of the cutaneous system. Along the “ lateral cranio-facial line,” in the 
