804 MR EDWARD J. BLES ON THE 
cement organ at the later stage. And the postero-lateral horns of this crescentic area 
(fig. 8A) are carried inwards towards the median dorsal line as the roof of the fore-brain 
closes in and give rise to the transverse band of cells I have called the frontal gland. 
This will be dealt with in detail later. We shall see that the area shown in fig. 9. 
enclosed by the line of the frontal gland and the upper edge of the cement organ is 
the part from which the anterior organs of the face develop, viz.—the stomodceum 
and the nasal pits. At this stage the egg has just begun to elongate in the direction 
of the future axis of the body. 
In fig. 10 an early stage in the elongation of the larva is represented, and it will be at 
once observed that the dorsal concavity which is so marked a feature of corresponding 
stages in all such forms as Rana, Bufo, Hyla, Bombinator, and Discoglossus is not only 
absent here, but there is a distinct convexity of the dorsal contour in side view—the 
two ends of the embryo are slightly bent ventralwards, so that there is a shallow ventral 
concavity. It is this feature in its general appearance which in early embryonic stages 
of Xenopus recalls rather a Urodelan embryo than one of the familiar Anuran embryos. 
The difference is due mainly to the fact that while the embryo of the typical Phanero- 
glossa owe their increase in length from the beginning to the elongation of the tail and 
sometimes of the head also, the abdominal region remaining short, the embryo of 
Xenopus for sometime grows in length in the trunk region, the tail remaining short 
and stunted. Connected with the checked growth of the trunk in the forms mentioned | 
above is the persistence of the anus in its position high up on the posterior surface of 
the ege, which involves the outgrowth of the tail, so as to make it sprout not directly 
backwards but obliquely upwards at an obtuse angle with the long axis of the body. In 
Xenopus, on the other hand, the growth takes place in such a way that the anus is 
swung round from its equatorial position on the spherical egg to a ventral position in 
the elongating embryo, as indicated in fig. 10. As indicated in figs. 10 and 12-14, the 
tail remains a mere stump, while the trunk elongates considerably. A form of embryo 
results, bearing considerable resemblance in the proportions of its main regions to 
Urodele jarvee and also to young Dipnoan larvee, where the anus is placed close to the 
posterior end of the body at the root of a very short tail. This short-tailed, long- 
bodied phase of the development of Xenopus may, with some confidence, be looked 
upon as primitive. 
Returning to stage 10, it will be seen that the region of the spinal cord is pinched 
up, as it were, from the ventral part of the trunk, and the fore- brain swells out the tip of 
the head. Behind the fore-brain is an elevation of the whole branchial region. ‘There 
is an accumulation of yolk at the posterior end producing a swelling out of that part. 
The fin-fold has not made its appearance. The cement organ is beginning to assume its 
characters, showing a compact group of cells with densely pigmented inner ends and 
outer ends filled with a clear mass of secretion. At the extreme anterior end is placed 
the frontal gland, seen better in the frontal view (fig. 10a). This shows how the frontal 
gland has become continuous with the pigmented band of cells along the sagittal line. 
