10 EMBRYOLOGY. 



etc.) ; more usually it results in the production of females only, and 

 there are very often in the Arthropoda a series of successive gene- 

 rations of females all producing ova which develope partheno- 

 genetically into females ; eventually however, usually in direct or 

 indirect connection with a change of food or temperature, or other 

 conditions, ova are formed which give rise without fertilization both 

 to males and females. 



The true asexual modes of reproduction amongst the Metazoa 

 consist of fission and gemmation. Gemmation is by far the most 

 widely disseminated of the two. Various as are the methods in 

 which it takes place, it seems nevertheless that cells derived from all 

 the germinal layers, and very frequently from all the important 

 organs of the adult, assist in forming the bud. Into the details of 

 the process, which require in many points a fuller elucidation, it 

 is not my purpose to enter. 



Gemmation is a far commoner occurrence amongst the simpler 

 than amongst the more highly organised forms. It appears to have 

 been superadded to the sexual mode of reproduction quite inde- 

 pendently in a number of different instances. 



While there is no difficulty in understanding how gemmation may 

 have started in such simple types as the Ccelenterata, the manner 

 in which it first originated in certain highly organised forms, as for 

 instance the Ascidians, is somewhat obscure, but it seems probable 

 that it began with the division of the developing germ into two or 

 more embryos, at a very early stage of growth. 



Such a division of the germ is, as has been shewn by Kleinenberg, 

 normal in Lumbricus trapezoides 1 and Haeckel has shewn that an 

 artificial division of the germ in the Siphonophora leads to the 

 development of two individuals. It has been pointed out by 

 various naturalists that the production of double monsters is 

 often a phenomenon of the same nature. While it is next to 

 impossible to understand how production of a bud could commence 

 for the first time in the adult of a highly organised form, it 

 is not difficult to form a picture of the steps by which the fission 

 of the germ might eventually lead to the formation of buds in the 

 adult state. 



The coexistence of sexual reproduction with normal asexual multi- 

 plication, or with parthenogenesis, has led to a remarkable pheno- 

 menon in- the animal kingdom known as alternations of generations 2 . 



For the details of the various types of alternations of generations, 

 and their origin, the reader is referred to the body of the work ; but 

 a few general remarks on the nature and origin of the process, and 

 on its nomenclature, may conveniently be introduced in this place. 



1 The case of Pyrosoma, which might be cited in this connection, is probably 

 secondary. 



2 For an excellent account of this subject, vide Allen Thomson's article Ovum in 

 Todd's Ci/rloptftlin. The metamorphosis of the Echinoderras included under this head 

 in Thomson's Article is now known not to be a proper case of alternations of generations. 



