308 
Part III. — Eighth Annual Report 
The foundations of two of the gili-clifts (g.c.), and traces of a third are 
also seen. Some 46 mesoblastic somites (structures which give rise sub- 
sequently to most of the muscles and also to much of the deeper portion 
of the skin) can be made out. In front of these somites 2 others sub- 
sequently appear, and behind them the formation of new ones goe3 on 
until upwards of 130 are present. In this stage the pronephros and some 
portion of the segmental dust are already present. In the next stage 
represented (fig. 14) the embryo had grown considerably in length. It 
measured 7*5 mm. 
Four gill-clifts (g.c.) were present, and a fifth was in course of formation. 
While the preceding embryo corresponds to stage H of Balfour, this one 
is from the latter half of the stage J, as defined by Van Wyhe. Some 68 
somites could be counted. The figure shows how these decrease in size 
from before backwards. The neurenteric canal is still present. It 
apparently persists as long as mesoblastic somites are being formed. 
In the brain no great changes can be noted as regards external appear- 
ance. In the eye the formation of the lens has commenced. The ear 
shows no appreciable difference from the preceding stage. 
The mouth opening is not yet formed, but the oral part of the hypo- 
physis cerebri (hy.) is seen as an involution of epiblast towards the floor of 
the brain. Five well-marked gill-slits are characteristic of the following 
stage ; the need of a limitation of the number of figures has prevented 
its representation, and, as it is not shown, I shall not describe it in this 
paper. 
The embryo of the following figure (fig. 15) had the full complement 
of 6 gill-clifts, and more than 100 somites were present. Their exact 
number could not be determined, owing to a slight malformation and 
curling-up of the tail of the embryo. In other respects the embryo was 
perfectly normal. 
Only the anterior end of the embryo is depicted. It measured fully 
double the length of the part shown in fig. 14. 
The mouth is formed, and all the gill-clifts are freely open towards the 
exterior. The most anterior (the future spiracular clift) is already under- 
going constriction in its ventral portion. On the posterior margins of 
the hyoid, first, second, and third branchial arches, the first rudiments of the 
external gill-filaments (e.g.) (about which more is written in another 
section of this paper), are seen as small processes. These grow larger and 
larger, blood-vessels are also developed with them and within them (in the 
form of a looped tube in each filament), the number of these filaments 
increases, and shortly there projects a bunch of these filaments from each 
of the five posterior clifts. The spiracular cleft never possesses them. 
The heart (ht.) is now very obvious, and at this stage, and earlier, actively 
performs its functions. Along the umbilical cord (u.c), the vessel which 
brings blood back from the yolk-sac is seen passing to the heart, and 
from the anterior end of the latter organ the branchial aorta (b.a.) is 
shown lying below the gill-clifts. 
The lens (I) of the eye has separated from the ectoderm, the true retina 
(r) is in course of development, and the outer layer (pr.) of what was the 
secondary optic vesicle, is already pigmented in some places ; it is the 
future ' pigment layer of the choroid.' The ear (a.u.\ though larger, 
has undergone but little differentiation ; the auditory nerve (vm.) is seen 
applied to it. In front of this nerve, and fused with it, is seen the large- 
facial nerve (vn.) passing to the spiracular cleft, with the upper wail of 
which it is fused. At the point of this fusion (a fusion with a patch of 
sensory epithelium over each of the gill-clefts) the foundation of the future 
organs of the lateral line or lateral sense organs takes place in the case of each 
