227 



in which the foetus is kept during the whole time of utero-gestation, 

 and upon the influence of the bodily and mental affections of the 

 mother upon the child ; in further illustration of which, several in- 

 stances are detailed in proof of the descent of various peculiarities of 

 the mother to the offspring. 



Observations on the Changes the Ovum of the Frog undergoes during 

 the Formation of the Tadpole. By Sir Everard Home, Bart. V.P.R.X. 

 Read November 25, 1824. [Phil. Trans. 1825, p. 81.] 



The ova of the Frog, when examined in the ovaria, consist of dark 

 coloured vesicles, which acquire a gelatinous covering on entering the 

 oviduct, and are completely formed by the time they reacli the cavi- 

 ties in which the oviducts terminate, and during their expulsion from 

 which they receive the male influence ; after this, the contents of the 

 ovum, previously fluid, coagulate and expand, the central part being 

 converted into brain and spinal marrow, while in the darker sub- 

 stance of the egg the heart and other viscera are formed. The mem- 

 brane forming the vesicles being destined to contain the embryo when 

 it has become a tadpole, enlarges as the embryo increases, and may 

 be said to perform the office both of the shell and its liniflg mem- 

 brane in the pullet's egg, serving as defence and allowing aeration. 

 The black matter which lines the vesicle probably tends to the de- 

 fence of the young animals from the too powerful influence of the 

 solar rays, frogs' spawn being generally deposited in exposed situ- 

 ations. Sir Everard observes, that in the aquatic Salamander, an 

 animal whose mode of breeding closely resembles the frog, this ni- 

 grum pigmentum is wanting ; but that that animal deposits its eggs 

 within the twisted leaves of water plants, which afford them an equi- 

 valent protection. 



A general Method of calculating the Angles made by any Planes of 

 Crystals, and the Laws according to which they are formed. By the 

 Rev. W. Whewell, F.R.S. Fellow of Trinity College, Cambridge. 

 Read November 25, 1824. [Phil. Trans. 1825, p. 87.] 



The author, after stating the inconsistencies, inelegancies, and im- 

 perfections of the received notation for expressing the planes of a 

 crystal, and the laws of decrement by which they arise, and of the 

 usual methods of calculating their angles, explains the object of the 

 present paper, which is to propose a system exempt from these in- 

 conveniencies, and adapted to reduce the mathematical portion of 

 crystallography to a small number of simple formulae, of universal 

 application. According to the method here followed, each plane of 

 a crystal is represented by a symbol indicative of the laws from which 

 it results, which, by varying only its indices, may be made to repre- 

 sent any law whatever ; and by means of these indices, and of the 

 primary angles of the substance, we may derive a general formula 

 expressing the dihedral angle contained between any one plane re- 



Q -2 



