146 
ME. W. K. PAEKEE ON THE STEITCTUEE AND 
the Tadpole and like that of the Myxinoid, which it represents when its larval co n 
dition is perfect. The fronto-nasal process is now transformed into the large upper lip, 
with its unequal halves separated by a small sulcus. 
If the fore part of an embryo at this stage be cloven vertically, we shall see what is 
depicted in Plate III. fig. 12. 
The simple anterior cerebral vesicle (fig. 12, C 1) is now evidently composed of three 
lobes, namely, a small one in front, the “ rhinencephalon ” (fig. 12, 1“), then a large 
lobe, more than hemispherical, the prosencephalon (C 1*), and above and behind this a 
lobe which is compressed below; this is the “deutencephalon” (C l c ). 
Behind this third division of the anterior vesicle is the middle, and behind it the third. 
These are now arranged in a less curved line ; for the mesocephalic flexure is becoming 
less day by day. This section shows the heart ( h .) and pericardium ( pc.d .), and also that 
the yelk-mass (y.) is retreating, the pharyngeal cavity having grown much larger and the 
thoracic organs more differentiated. Anteriorly, we see more clearly defined labials 
(u.l., l.l .) ; and above the lower labial a bud of cartilage is seen ascending from the lower 
end of the first preoral arch ; this is the first definite appearance of Meckel’s cartilage 
( m.k .). A side view, with the skin dissected away, shows how far the facial arches have 
advanced since the last stage (compare Plate III. fig. 3 with Plate IV. fig. 1). The 
whole of the facial wall up to the yelk-mass is occupied by the moieties of the closely 
packed visceral arches. 
This series now shows a seventh, the last, branchial ( br . 4) ; it is the smallest; and the 
trabecula, which was the most massive, begins now to yield to the second and third. 
These bars have the same general form, and are much more elegant than in the first 
stage; they all curve inward both above and below, and are bowed forward both above 
and below ; at their middle they are gently arcuate in the same direction (Plate IV. 
fig. 1). 
The first pair, or preoral, have begun to adapt themselves more perfectly to the floor 
of the brain-sac, but they still largely retain their parallelism with the next, or first 
postoral pair. Below, the first postoral has not only gained a Meckelian bud , but it lies 
also at a further distance from the one behind it ; this is caused principally by a com- 
mencing bend backwards of the lower half of the second postoral, preparatory to the 
subdivision of this pair of rods into the hyo-mandibnlar and cerato-hyal pieces. This 
arch also has now grown to be much larger than its successors (compare Plate III. fig. 3 
with Plate IV. fig. 1) ; it also projects further outward, the skin covering it becoming 
the gill-cover (Plate III. fig. 10, op., and Plate IV. fig. 5, op.). The top of this third 
arch is immediately below the fore end of the auditory capsule (cm.), which is still 
an open sac. The remaining four arches form a series regularly decreasing in size, 
and their outer surface becomes more and more buried in the neck of the embryo (see 
also horizontal section, Plate IV. fig. 5, br. 1-4). 
All these seven bars are still totally free, both from each other and the surrounding 
organs : the free, incurved upper end of the first is postorbital in position ; the next 
