426 
MESSRS. E. M. BALFOUR, AND W. N. PARKER ON THE 
through the central part of the organ. The whole organ has the characteristic 
structure of a simple lung. 
The liver (/?’.) consists of a single elongated lobe, about 32 centims. long, tapering 
anteriorly and posteriorly, the anterior half being on the average twice as thick as the 
posterior half. The gall-bladder ( g.b .) lies at its posterior end, and is of considerable 
size, tapering gradually so as to pass insensibly into the bile duct. The hepatic duct 
(Jip.d.) opens into the gall-bladder at its anterior end. 
The spleen (s.) is a large, compact, double gland, one lobe lying in the turn of the 
intestine immediately above the spiral valve, and the other on the opposite side of the 
intestine, so that the intestine is nearly embraced between the two lobes. 
11. — Development. 
We have already described in detail the first formation of the alimentary tract so 
far as we have been able to work it out, and we need only say here that the anterior 
and posterior ends of the canal become first formed, and that these two parts gradually 
elongate, so as to approach each other ; the growth of the posterior part is, however, 
the most rapid. The junction of the two parts takes place a very short distance 
behind the opening of the bile duct into the intestine. 
For some time after the two parts of the alimentary tract have nearly met, the 
ventral w T all of the canal at this point is not closed; so that there is left a passage 
between the alimentary canal and the yolk-sac, which forms a vitelline duct. 
After the yolk-sac has ceased to be visible as an external appendage it still persists 
within the abdominal cavity. It has, however, by this stage ceased to communicate 
with the gut, so that the eventual absorption of the } T olk is no doubt entirely effected 
by the vitelline vessels. At these later stages of development we have noticed that 
numerous yolk nuclei, like those met with in Teleostei and El as mobranchii, * are still 
to be found in the yolk. 
It will be convenient to treat the history of sections of the alimentary tract in front 
of and behind the vitelline duct separately. The former gives rise to the pharyngeal 
region, the oesophagus, the stomach, and the duodenum. 
The pharyngeal region, immediately after it has become established, gives rise to a 
series of paired pouches. These may be called the branchial pouches, and are placed 
between the successive branchial arches. The first or hyomandibular pouch, placed 
between the mandibular and hyoid arches, has rather the character of a double layer 
of hypoblast than of a true pouch, though in parts a slight space is developed between 
its two walls. It is shown in section in Plate 24, fig. 43 (Aw.), from an embryo of 
about 10 millims., shortly before hatching. It does not appear to undergo any further 
development, and, so far as we can make out, disappears shortly after the embryo is 
hatched, without acquiring an opening to the exterior. 
# For a history of similar nuclei, vide ‘ Comp. Embryol.,’ vol. ii., chapters iii. and iv. 
