Philosophical TraiisactioJis. 295 



iii. Observations on the Changes the Ovum of the Frog undergoes 

 during the Formation of the Tadpole, By Sir E. Home, Bart., 

 V.P.R.S. 



In the year 1S22, the author laid before the Society a series of 

 observations on the progress of the formation of the chick in the 

 egg of the pullet, illustrated by drawings from the pencil of Mr. 

 Bauer, showing that in the ova of hot-blooded animals the first 

 parts formed are the brain and spinal marrow. He has now 

 brought forward a similar series on the progress of organization 

 in the ova of cold-blooded animals, illustrated in the same manner 

 by microscopical drawings made by the same hand. 



The ova of the frog, which have been selected for this investi- 

 gation, are found to have no yelk. If we examine these ova in 

 the ovaria in which they are formed, we find them to consist of 

 small vesicles of a dark colour ; when they enter the oviducts they 

 enlarge in size, and acquire a gelatinous covering, which increases 

 in quantity in their course along those tubes ; but the ova car. 

 neither be said to have acquired their full size nor to have re- 

 ceived their proportion of jelly, till they arrive at a cavity close 

 to the termination of each oviduct, formed by a very considerable 

 enlargement of those tubes, corresi)onding in many respect? to 

 the cloaca in which the pullet's egg is retained till the shell be- 

 comes hard. 



When the ova are deposited in these reservoirs, they become 

 completely formed, and in a state to be impregnated by the male 

 influence, which is applied to them in the act of their expulsion. 

 A8 they are pressed upon each other, by being confined in a small 

 space, the gelatinous covering takes an hexagonal figure, in the 

 centre of which is the ovum. 



Immediately after impregnation there is no change in the ap- 

 pearance of the jelly, nor of the vesicle contained in it, in this 

 respect corresponding exactly with what happens to the pullet's 

 egg. The first change that is produced towards the formation of 

 an embryo is, the contents of the vesicle expand, its form changes 

 from that of a sphere to an oval, and when cut through its con- 

 tents are no longer fluid. In the act of coagulation, the central 

 portion becomes of a lighter colour than that which surrounds it, 

 swells out in the middle, and there is a distinct line by which the 

 two portions are separated from one another ; the central part, in 

 its future changes, is converted into brain and spinal marrow, and 

 after these organs have acquired a defined outline, the heart and 

 dther viscera are seen forming in the darker substance. 



The membrane that forms the vesicle which is destined to con- 

 tain the embryo when it has become a tadpole, has a power of en- 

 largement as the embryo increases in size, and then performs the 

 oflice both of the shell and of the membrane that lines it in the 



