EMBRYOLOGY 203 
and swellings at the head-end into the Brain. Its walls are 
entirely formed of epiblast. 
The tube of the alimentary canal and that of the general body- 
cavity are formed in a totally different way. They are, broadly 
speaking, the result of the junction and coalescence of the funda- 
mental embryonic folds, the head-, tail-, and lateral folds. It is 
obvious that the folding in of a single sheet of tissue, such as we 
hitherto considered the blastoderm tube, can only result in the 
production of a sac with a single cavity, and would not explain 
the formation of the double tube. The blastoderm, however, soon 
splits throughout its greater part into a double sheet, an upper and 
a lower leaf. In the neighbourhood of the axis or future vertebral 
column, beneath the neural tube, this cleavage is absent. In fact 
the cleavage begins at some little distance on either side of the axis, 
and thence spreads through the mesoblast horizontally to the 
margin. ‘The upper leaf or half of the mesoblast remains united 
with the epiblast, and from its forming the body-walls, is called the 
somatopleure ; the lower half of the mesoblast, together with the 
hypoblast, forms the alimentary canal and its tributary viscera, and 
is therefore called the splanchnopleure. ‘The space between the two 
pleura or flaps is the general body- or pleuro-peritoneal cavity. 
This cleavage of the mesoblast into a somato- and splanchno- 
pleure is uot confined to the region of the embryo, but extends in 
time over the whole of the yolk-sac. Hence the yolk-sac comes 
ultimately to have an inner splanchnopleuric and an outer somato- 
pleuric coat, and since, as we have seen above, the embryonic sac is 
connected with the yolk-sac by a continually narrowing hollow stalk, 
this stalk must be likewise double, consisting of a smaller inner 
stalk within a larger and outer one. ‘The narrow space between 
these two investments of the yolk-sac is continuous with the pleuro- 
peritoneal cavity. Long before hatching the inner stalk becomes 
obliterated, so that the material of the yolk can no longer pass 
directly into the alimentary canal (the walls of which were con- 
tinuous with the walls of the inner stalk), but has to find its way 
into the body of the chick by absorption through the blood-vessels, 
which by this time have spread over the yolk-sac. ‘The outer or 
somatic stalk remains widely open for a long time as a thin and 
insignificant continuation of the somatopleure. When in the last 
days of incubation the greatly diminished yolk-sac, with its 
splanchnic investment, is withdrawn into the rapidly enlarging 
abdominal cavity of the embryo, the walls of the abdomen (them- 
selves somatopleuric) close in and unite without regard to the 
shrivelled, emptied, somatopleuric investment of the yolk-sac, which 
is cast off as no longer of any use. The place where this has 
happened is the outer umbilicus or navel, long visible on the middle 
of the belly of the young bird. Remnants of the stalk between the 
