808 MR EDWARD J. BLES ON THE 
organ (fig. 17c). In this position the tadpole remains for not more than thirty 
seconds ; the stretched part of the membrane then bursts, and the larva is shot out head 
foremost, as shown in fig. 18. In the same instant the vitellme membrane shrivels up | 
like a burst indiarubber toy balloon. The larva may remain motionless with its tail 
between the torn edges of the eeg membranes for one minute ; it then wrigeles and frees 
the tip of its tail, the tail swings round through an are of 180° and the larva is then 
seen hanging, as in fig. 19, by a thread of mucus to the still further shrunken ego mem- 
branes. This thread of mucus is the short thread which attached the larva to the inside 
of the vitelline membrane and now drawn out to the length of the larva. It is 
apparently pulled out when the tadpole bursts out of the membranes, but it is not 
shown in fig. 18, as it is hidden by the body. 
The following suggestions are offered as to the method of this hatching process. 
The larva attaches itself and fixes its position in relation to the vitelline membrane. 
Hach time the animal turns over it must necessarily straighten itself, and in so 
doing the frontal gland touches the vitellme membrane, and smears it with a 
little of the secretion of the frontal gland. The secretion is of a different nature 
to the secretion of the cement organ; it acts upon the vitellme membrane and 
softens it. The softened patch is distended by the pressure of the fluid in the 
turgid ego, and the head of the larva sinks into the pouch which is formed. When 
the frontal gland comes into contact with the vitelline membrane in the last 
stages of hatching, the softening process is hastened, and when the larva’s head 
is pressed against the pouched-out membrane, the fluid pressure in the ege must 
act, not on the membrane, but on the body of the larva. The fact that the 
softening process only goes on opposite the anterior surface of the head is shown 
by the vitellme membrane retaining its normal curvature at all other points, even 
so close to the head as opposite the neck (see figs. 174 and B). 
To test the hypothesis that the secretion of the frontal gland acts on the egg 
membrane, the following experiment was made. Four eggs out of six left 
unhatched at 12 p.M. on 27th August were hung up, so that the long axis ay 
(fic. 16) was vertical instead of horizontal, and the head of the larva uppermost. 
By grasping with fine forceps a little of the jelly at the pole 2, and drawing it 
out of the water against the glass of the tank, the eggs could be fixed in this 
position. Of the four eggs slung up in this way, none were hatched at 6.30 P.M. ; 
the two eggs left in the normal position hatched out the larve between 1 P.M. 
and 2 p.m. At 6.30 p.m. the four eggs were returned to the normal position, and 
within half an hour all were hatched. The larvae were thus prevented from 
hatching for about five hours. 
The reason appears to be this. When the egg is carefully revolved into the 
new position, the larva remains attached, but slides down towards the pole y; 
the head is consequently carried away from the egg membrane near the pole 2, 
and when the larva moves, it is easy to see that the head apex, where the frontal 
