There has been a good bit of work on this, though there are still many unanswered questions. Below, I have sent two references. The first is a classic work from some time ago that gives some general experimental data. The second is a bit more recent, giving specific information about vitellocyte function. The referene lists should help you with more of the older literature. Little research is being done today on this topic.

Wilson RA. 1968. The hatching mechanism of the egg of Fasciola hepatica L. Parasitology. 1968 Feb; 58(1):79-89.

Schmidt J. 1998. Glycan vesicle formation in vitellocytes and hatching vacuoles in eggs of Echinostoma caproni and Fasciola hepatica (Digenea). Tissue Cell. 1998 Aug; 30(4):416-26.

Thank very much for your information and interest in our investigation

My preliminary research will work on Fasciola gigantica, larval stage in experimental snails.

Dear,

after collection the eggs either from gall bladder of infected livestock at the abattoir of from the flukes, they are incubated at 20 or 25 C at complet darkness. it is a better to divide the eggs in many pots, and each one is covered by aluminum paper. after 20 or 15 days according to the incubation temperature, a pot can subject to an artifacial light to enhance miracidia shedding. you could verify one of the following articles:

Dar, 2002, parasitol res 88, 364-367

Dar, 2003, parasitol res, 89, 185-187

Hatching depends on several abiotic factors.Temperature has great significance.In optimum temperature conditions mass hatching of miracidia will ensure.Also light and osmotic pressure of medium have stimuleted effect

I found in the literature:

Hatching mechanism of egg of Fasciola hepatica remains a subject of discussion. Three main hypotheses could be delimited:

First, Thomas (1883) thought that the larvae actively left the egg. This process is accompanied by the thrusting of the operculum by means of muscular effort.

Later, Rowan (1956, 1957) denied the role of muscular activity. He thought some environmental factors activate the hatching: (a) light intensity; (b) "hatching enzyme" a proteolytic enzyme (produced by the miracidium itself) which attacks the "cement" holding the operculum, and causes the destruction of the yolk membrane. (c) All this results in a change and an emerging difference in the osmotic pressure of the fluid inside the egg and in the environment pushing the miracidium out of the egg.

Third hypothesis ( Wilson, 1968) which was later to get a substantial experimental support : the larval hatching takes place by means of an abrupt increase in hydrostatic pressure inside the egg. The cushion is not simply a colloid mass but a complicated organized fibrillar mucoid-protein complex, the volume of which considerably increases during hydration. The larvae often leaves the egg with the remains of the enveloping yolk membrane which is finally destroyed in the environment.

This process is triggered by the external signal factors, activating the larva. It is the larva that initiates the process of cushion hydration, presumably by means of substances secreted by some glands, and alters the permeability of the yolk membrane and other barrier structures of the egg-shell. In particular, special hatching glands have been described in some miracidia.

References:

Galaktionov,K.V. and Dobrovolskii, A.A. The biologiy and evolution of trematodes. Edited by B. Fried and T.K.Graczyk. Kluwer Academic Publishers Dordrecht/Boston/London. 2010.

Rowan, W.B. The mode of hatching of the egg of Fasciola hepatica. Exp Parasitol 1956; 5/118-37

Rowan, W.B. The mode of hatching of the egg of Fasciola hepatica 2. Colloidal nature of the viscous cushion. Exp Parasitol 1957; 6/131-42

Thomas,A.P. The life history of the liver flucke. Quart J Micr Sci 1883; 23:90-133

Wilson, 1968 - The hatching mechanism of the egg of Fasciola hepatica L. Parasitology 1968; 58: 79-89