134 



ANATOMY AND PHYSIOLOGY OF 



for, if it be supposed that this subdivi- 

 sion has taken place when the develop- 

 mental process has advanced no further 

 than in a Hydra or a Planaria, it seems by 

 no means impossible that each part might, 

 as in those creatures, advance in its devel- 

 opment up to the attainment of its com- 

 plete form. 



" There are many tribes, both of Plants 

 and Animals, in which multiplication is 

 effected not only artificially but sponta- 

 7ieoi(slt/, by the separation of parts, which, 

 though developed from the same germ in 

 perfect continuity with each other, are capa- 

 ble of maintaining an independent exist- 

 ence, and which, when thus separated, take 

 rank as distinct individuals. This process, 

 which is obviously to be regarded, no less 

 than the preceding, as a peculiar manifes- 

 tation of the ordinary operations of Nu- 

 trition, may take place in either of four 

 different modes — 1. In the lowest Cellular 

 Plants, and the simplest Protozoa, every 

 component cell of the aggregate mass that 

 springs from a single germ, being capable 

 of existing independently of the rest, may 

 be regarded as a distinct individual; and 

 thus every act of gi'owth which consists in 

 the multiplication of cells, makes a corre- 

 sponding augmentation in the number of 

 individuals. 2. In many organisms of a 

 somewhat higher type, in which the fabric 

 of each complete individual is made up of 

 several component parts, we find the new 

 growths to be complete repetitions of that 

 from which they are put forth ; afid thus 

 the composite organism presents the sem- 

 blance of a collection of individuals united 

 together, so that nothing is needed but the 

 severance of the connection, to resolve it 

 into a number of separate individuals, each 

 perfect in itself. The most characteristic 

 example of this is presented by the Hydra, 

 which is continually multiplying itself after 

 this fashion ; for the buds or ' geramsB ' 

 which it throws off are not merely struc- 

 turally but functionally complete (being 

 capable of seizing and digesting their own 

 prey), previously to their detachment from 

 the parent. 3. In by far the larger propor- 



tion of cases, on the other hand, the 

 ' gemma ' does not possess the complete 

 structure of the parent, at the time of its 

 detachment, but is endowed with the ca- 

 pacity for developing whatever may be 

 deficient. Thus, the bud of a Phanero- 

 gamic Plant possesses no roots, and its 

 capacity for independent existence depends 

 upon its power of evolving those organs. 

 On the other hand, the 'zoospore' of an 

 Ulva or a Conferva is nothing else than a 

 young cell, from which the entire organism 

 is to be evolved after it has been set free; 

 and, even in the • bulbels ' of the Mar- 

 chantia, the advance is very little greater. 

 The ' bulbels ' of certain Phanerogamic 

 plants, however, bear more resemblance to 

 ordinary buds. 4. In the preceding cases, 

 the organism which is developed by this 

 process resembles that from which it has 

 been put forth ; but there are many cases 

 in which the offset differs in a marked de- 

 gree from the stock, and evolves itself into 

 such a different form that the two would 

 not be supposed to have any mutual rela- 

 tion, if their affinity were not proved by a 

 knowledge of their history. Sometimes 

 we find that the ne^v individual thus bud- 

 ded off is in every respect as complete as 

 that from which it proceeded, though de- 

 veloped upon a different type ; but in other 

 instances it is made up of little else than 

 a generative apparatus, provided with loco- 

 motive instruments to cany it to a distance, 

 its nutritive apparatus being very imperfect. 

 Of the first, we have an example in the 

 development of Medusse from the Hydroid 

 Polypes ; and of the second in the peculiar 

 subdivision of certain Annelida, hereafter 

 to be described. Now it is obvious that, in 

 this process, no agency is brought into 

 play that differs in any essential mode from 

 that which is concerned in the ordinary nu- 

 tritive operation. The multiplication of 

 individuals is performed exactly after the 

 same fashion as the extension of the parent 

 organism ; and the very same parts may 

 be regarded as organs belonging to it, or as 

 new individuals, according to their stage of 

 development, and the relation of depen- 



