LIFE-HISTORY OF XHNOPUS LA&VIS, DAUD. 807 
Hatching.-—The larva of Xenopus hatches forty-eight hours after the egg is laid, 
when the eggs are kept undisturbed in a constant temperature of 22°C. I found, for in- 
stance, that a batch of some hundreds of eggs, laid on the night 24th-25th August 1903, 
had all hatched on the morning of 27th August, with the exception of twenty or thirty. 
As the larvee might be expected to emerge from these during the course of the day, they 
were placed for observation in a little tank kept at a temperature of 22° and were 
watched from the side through a horizontal binocular microscope (Zeiss’ Braus-Driiner 
model) under a low power. The view of the ege obtained in this way is represented in 
fic. 16, Plate III. ‘The jelly on the surface is not shown ; a very thin layer of it covers 
the vitellime membrane, the thinness being partly due to the way in which the vitelline 
membrane has swollen up, the jelly having now to cover a much larger surface. At an 
early stage the membrane loses its spherical form and becomes ellipsoidal (see fig. 12) ; it 
is elastic and becomes swollen up by the pressure of its fluid contents. It is easy to test 
this by tearing a hole in the membrane; a jet of the fluid is forced out through the 
hole and the membrane collapses and shrinks. The egg is, then, in a condition of tur- 
gidity, and this forms a factor in the hatching process. The larva lies on the lowest part 
of the vitelline membrane, the side of its body in contact with it and roughly parallel to 
the long axis of the egg, which is always horizontal. In this position the larva is attached 
by the secretion of its cement organ to the vitelline membrane about two hours before 
the hatching occurs. Before attaching itself it seems to have sunk down to a position 
of equilibrium with its centre of gravity over the lowest point in the curve of the egg 
membrane. ‘This is indicated by the position of the tip of the tail, which is always at a 
higher level than the head. The surface of the larva is richly ciliated and the fluid in 
the egg membrane is kept in rapid rotation, the current over the body of the larva 
passing from head to tail. The larva lies perfectly still, except that every ten to fifteen 
minutes it turns over from one side to the other. It will be seen from fig. 16 that the 
head of the larva only touches the vitelline membrane where the surface of the eye 
rests upon it; the cement organ is not itself in contact, as a short string of secretion 
passes from the gland to the membrane. The first sion that hatching is about to take 
place is a slight bulging outwards of the vitelline membrane opposite the head of the 
larva. In the course of the next five minutes the membrane under the anterior part of 
the head softens and the head sinks into the soft place, the membrane partly moulding 
itself to the contours of the head and partly bulging beyond the head, as drawn in fig. 
174, Plate III. It will be observed that in this position the extreme anterior end of the 
head where the frontal gland is situated is not actually touching the vitelline membrane, 
although very close to it. In another five minutes the membrane has moulded itself by 
a further softening to the anterior contour of the head (see fig. 178), and now the 
frontal gland touches the vitelline membrane. When this stage is reached the hatching 
is rapidly completed. During the next three minutes the pouch of vitelline membrane 
in which the head lies distends more and more, until an imaginary line drawn in the 
original smooth curve of the membrane would pass through the middle of the cement 
