ON ORGANIZED STRUCTURES IN GENERAL. 101 



contribute to its progressive elongation. Of the transitory duration of the 

 Assimilating cells, we have an obvious proof in the "fall of the leaf;" which 

 takes place at intervals (alike in evergreen and deciduous species), to be fol- 

 lowed by the production of a new set of cells, having similar functions. And 

 the Secreting cells have usually a like transitory duration ; being destined to 

 give up their contents by the rupture or liquefaction of their walls, whenever 

 called upon to do so, by the demand set up in the growing parts of their 

 neighbourhood, for the peculiar products they have set apart. 



109. Not only are the proper organic functions of all Plants thus dependent 

 upon the agency of cells ; but their Reproduction is likewise. In the lowest 

 tribes of the Cryptogamia, where each cell is an independent individual, every 

 one has the power of preparing within itself the reproductive germs, from 

 which new generations may arise. In the higher tribes, on the other hand, 

 the general principle of the division of labour, which separates the absorbing, 

 assimilating and secreting cells, involves also the setting apart of a distinct set 

 of cells for the preparation of the reproductive germs ; these cells are known 

 in the Cryptogamia as spores, and in the Phanerogamia as pollen-grains. In 

 the higher Plants we find a complex apparatus superadded ; for the purpose 

 of aiding the early development of these germs, by supplying them with nu- 

 triment previously elaborated by the parent; yet still this operation is of a 

 purely accessory kind, and the essential part of the process remains the same. 



110. Now we shall find that, although the fabric of Animals appears to be 

 formed on a plan entirely different from that of Plants, and although the ob- 

 jects to be attained are so dissimilar, there is a much greater accordance 

 amongst their elementary parts, than might have been anticipated. The starting- 

 point of both is the same ; for the embryo of the Animal, up to a certain grade 

 of its development, consists, like that of the Plant, of nothing else than an aggre- 

 gation of cells (Plate I., Fig. 15). And amongst the lowest tribes of animals, as 

 well as among certain of the highest tribes that retain many embryonic peculi- 

 arities, even in the adult condition, (such as the curious Jlmphioxus or Lancelot,) 

 we find a great proportion of the complete fabric to be possessed of a similar 

 constitution. In most of the higher animals, however, Ave find that a large 

 proportion of the fabric consists of tissues in which no distinct trace of a cel- 

 lular origin is apparent ; and it has been only since improved powers of ob- 

 servation have been brought to bear upon their analysis, and more especially 

 since they have been examined, not only in their complete state, but in the 

 course of their development, that they have been reduced to the same category 

 with the tissues of Plants and of the lower Animals. Other tissues, which 

 are peculiar to Animals, cannot be referred to the same origin ; but these will 

 be found to have a grade of organization even lower than that of simple iso- 

 lated cells, and to be referrible to the solidification of the plastic or organizable 

 fluid prepared by the assimilating cells, and set free by their rupture. We 

 shall find, however, that (as in Plants) all the tissues most actively concerned 

 in the Vital operations, retain their original cellular form ; and we shall be 

 able to refer to distinct groups of cells in the bodies of Animals, not merely 

 the functions of Absorption, Assimilation, Respiration, Secretion, and Repro- 

 duction, which are common to them with Plants, but also those of Muscular 

 Contraction, and Nervous Action, which they alone perform. Before proceed- 

 ing to this investigation, however, it will be desirable to examine into the na- 

 ture of the original components at the expense of which the Animal fabric 

 is built up. Our knowledge of these is principally derived from the researches 

 which have been made into their character in Man and the higher Animals; 

 but there can be little doubt that they are common, with trifling modifications, 

 perhaps, to the entire kingdom. 



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