Royal Society. 155 



begins on one side near the equator, and soon forms a circular con- 

 striction which gives the yelk the figure of a dumb-bell, the longer 

 axis of which is the polar axis of the egg. The constriction travels 

 towards the germinal pole, and next produces a flask-shaped figure ; 

 this is at length lost by the constriction passing on, and the round 

 form is regained in about a minute. This wave reappears and 

 travels forward again without any distinct period of rest, and I have 

 seen these movements continue for forty-five minutes, though towards 

 the latter part of this period they are less distinct and more limited 

 in extent. The germinal mass has itself during these contractions, 

 which strongly resemble the peristaltic movements of the intestine, 

 undergone changes in form, and has increased in bulk and distinctness. 

 These movements are unaffected by weak galvanic currents. 



During the passage forward of each wave of contraction there is 

 an oscillation of the whole mass of the yelk, so that its germinal 

 pole passes once to the right and once to the left of the micropyle, 

 to which it at first corresponded. The plane of this oscillation may 

 be vertical, horizontal, or inclined, but always cuts the micropyle ; 

 it begins and ceases with the contractions already mentioned, and 

 would seem to be a mechanical result of them. 



For some time before cleavage begins, the only changes of form are 

 the appearance of wave-like elevations and depressions along the 

 under surface of the germinal mass, and its alternate concentration 

 and diffusion. Cleavage begins in about two hours after impreg- 

 nation ; no embryonic cell was observed before it began, nor in any 

 of the cleavage masses. 



The inner membrane is folded in during cleavage ; it is easily 

 seen thrown into folds at the cleft, and for this reason I do not 

 consider it a yelk-membrane, which term would be better applied to 

 the chorion. 



" On the Applicability of Gelatine Paper as a Medium for Colour- 

 ing Light." By Horace Dobell, Esq. 



The object of this communication is threefold. 



(1.) To point out the properties of a material called Gelatine 

 Paper, which render it applicable as a medium for colouring light. 



(2.) Through the means of gelatine paper, to introduce the use of 

 coloured light in the arts for the preservation of the sight of artisans. 



(3.) To introduce the use of gelatine paper for the relief of 

 persons suffering from impaired vision ; for the preservation of the 

 sight of travellers, and of all those who are much engaged in reading. 



This material was invented in 1829 by the late M. Grenet, of 

 Rouen, and was e..:.ibited by him in its present state of perfection 

 at the Great Exhibition of 18.51. But up to the present time it has 

 not been successfully applied to any more useful purposes than the 

 manufacture of artificial flowers, address-cards, tracing-paper, wafers, 

 wrappers for confectionary, and the like. 



It is commonly manufactured in sheets, measuring 22 inches in 

 length and 1 G inches in diameter, which are sold at a small price ; 

 but the sheets can as easily be made of any dimensions not ex- 

 ceeding those of which plate-glass is capable. It can be made of 



