LIFE-HISTORY OF XHNOPUS LAVIS, DAUD. 805 
The position of the neuropore is indicated by a few pigmented cells forming a short 
longitudinal streak exactly at the extreme anterior end of the animal. Between the 
neuropore and the cement organ is the region of the stomodceum. 
The next change in the external appearance of the embryo is due to the outgrowth 
of the median fin-fold of the tail. An early condition is shown in fig. 12. The trunk 
is slightly longer than before (stage of fig. 10), and the back of the embryo has become 
straighter. 
The growth in length of the embryo has stretched out the vitelline membrane into 
an ellipsoidal form. In the particular embryo drawn the cement organ happened to be 
larger than it is usual to find it at this stage. The line of the frontal gland is well 
defined ; it extends ventrally as far as the edge of the cement organ, and between it and 
that edge is enclosed a patch of deeply pigmented ectoderm. The portion of the 
median fin-fold to develop first is that bordering the tail region ; in fact, the extent of 
the tail region is fairly well defined by the limits of the fin-fold at this stage. The 
fold is deepest in the post-anal ventral part, extends round the posterior end of the 
embryo, and fades away just before reaching the dorsal surface. 
When the embryo has reached a length of 3°8 millimetres (fig. 13) the fin-fold has 
grown up along the dorsal surface and extends forwards as far as the part over the 
hind-brain, practically marking out the whole length of the organ. The fin is still 
deepest in the post-anal ventral portion. It is necessary at this stage to distinguish 
between a ventral post-anal abdominal part of the tail into which the posterior end of 
the yolk mass is drawn out, and a dorsal part which, it will be seen, grows out more 
vigorously and gives rise to the segmented, muscular part of the tail. In the head 
there is remarkably little indication externally of the developing eye, brain, and 
visceral arches. The position of the eye can just be discerned by the dark patch drawn 
in the figure; the mandibular arch is slightly raised above the general surface, and the 
posterior group of visceral arches is just discernible as another broad and gentle 
elevation on each side of the head. The line of the frontal gland is very distinct and 
obviously continuous with the cement organ at its lower ends. The cells of the cement 
organ are filled with clear secretion at their outer ends, and thus produce an appearance 
of a low translucent ridge running across the dark cement organ from side to side. No 
indications of nasal pits or of the stomodceum are to be seen. The myotomes are just 
beginning to be visible externally in the hinder trunk region. 
Further growth is shown in the embryo of fig. 14, drawn after removal from the 
egg. The tail has grown out more especially in its dorsal muscular part, and the fin- 
fold of the tail has now become widest along this muscular part. The post-anal 
abdomen has also been slightly drawn out in length by the growth of the tail. The 
dorsal fin has become higher, and now reaches forwards on the head as far as the hind 
level of the fore-brain. Numerous myotomes can now be seen in the living embryo. 
The eye is plainly seen and also indications of the lens thickening. A bulging behind 
the branchial region indicates the position of the heart; more dorsally the ear vesicle 
TRANS. ROY. SOC. EDIN., VOL. XLI. PART III. (NO. 31). 118 
