IN THE ANIMAL KINGDOM. 71 



when they are detached from their capsules and brought in 

 contact with the spermatozoa.* 



About the time of impregnation the germinal vesicle 

 is generally stated to disappear. In the Batrachia Mr. 

 Newport is inclined to think that it is ruptured by the 

 pressure of a brood of minute cells formed in its in- 

 terior, and its contents dispersed in the form of clear 

 spherules through the yolk or mass of oleo- albuminous 

 particles with which the ovum is filled. This may serve in 

 some way as a preparation for fecundation ; at all events he 

 is positive that in the ova of the newt and frog the disap- 

 pearance of the vesicle takes place before impregnation, and 

 not in consequence of it.*f* The first obvious result of this 

 act is the repeated cleavage or segmentation of the original 

 contents of the ovum, and the formation thereby of a mass 

 of cells or plastic spherules, out of which the embryo is de- 

 veloped either immediately, or with the intervention of some 

 of those diverse forms which occur in cases of so-called 

 Alternation of Generations. 



After impregnation, and when the process of segmenta- 

 tion is about to begin, a clear nucleated cell the " embryo 

 cell" is generally observed in the interior of the yolk. In 



* Gegenbaur, quoted by Huxley, Oceanic Hydozoa (Eay Soc.), p. 22. 

 These diversities in the mode of access of the Spormatozoa may be tabu- 

 la rly represented as follows : 



By a micropyle, in Echinodermata, Worms, Insects, Crustacea, 

 and Osseous Fishes (possibly also in some Eeptiles (HylaJ ; 

 By fissure of the wall of the ovum, in Mammalia ; (?) 

 By penetration through its substance, in Batrachia ; 

 By commixture before the ovum is coated over, in some Trema- 



toda and Nematoidea, and in Hydrozoa; 

 By dissolution of the investing membrane, in Lumbricus, and 



perhaps in some Hirudinei and Mollusca. 



The subject is noticed with some detail in Professor A. Thomson's article 

 (Ovum) in the supplementary volume of the Cyclopaedia of Anatomy and 

 Physiology. Also in the paper by M. Claparede, quoted above, 

 f Philosoph. Transactions (1851), p. 169. 



