18 THE ANATOMY OF INVERTEBRATED ANIMALS. 



as the equivalent of morphological unit, or some new term 

 must be invented to describe the latter. On the whole, it is 

 probably least inconvenient to modify the sense of the word 

 " cell." 



The histological analysis of animal tissues has led to sim- 

 ilar results, and to difficulties of terminology of precisely the 

 same character. In the higher animals, however, the modifi- 

 cations which the cells undergo are so extensive that the fact 

 that the tissues are, as in plants, resolvable into an aggrega- 

 tion of morphological units, could never have been established 

 without the aid of the study of development, which proves 

 that the animal, no less than the plant, commences its exist- 

 ence as a simple cell, fundamentally identical with the less 

 modified cells which are found in the tissues of the adult. 



Though the nucleus is very constant among animal cells, 

 it is not universally present ; and, among the lowest forms of 

 animal life, the protoplasmic mass which represents the mor- 

 phological unit may be, as in the lowest plants, devoid of a 

 nucleus. In the animal the cell-wall never has the character 

 of a shut sac containing cellulose ; and it is not a little diffi- 

 cult, in many cases, to say how much of the so-called " cell- 

 wall " of the animal cell answers to the " primordial utricle " 

 and how much to the proper " cellulose cell-wall " of the vege- 

 table cell. But it is certain that in the animal, as in the 

 plant, neither cell-wall nor nucleus is an essential constituent 

 of the cell, inasmuch as bodies which are unquestionably the 

 equivalents of cells — true morphological units — may be mere 

 masses of protoplasm, devoid alike of cell-wall and nucleus. 



For the whole living world, then, it results : that the mor- 

 phological unit — the primary and fundamental form of life- 

 is merely an individual mass of protoplasm, in which no fur- 

 ther structure is discernible ; that independent living forms 

 may present but little advance on this structure ; and that all 

 the higher forms of life are aggregates of such morphological 

 units or cells variously modified. 



Moreover, all that is at present known tends to the conclu- 

 sion that, in the complex aggregates of such units of which 

 all the higher animals and plants consist, no cell has arisen 

 otherwise than by becoming separated from the protoplasm 

 of a preexisting cell ; whence the aphorism, " Omnia cellula e 

 cettukV 



It may further be added, as a general truth applicable to 

 nucleated cells, that the nucleus rarely undergoes any consid- 

 erable modification, the structures characteristic of the tis- 



