THE MICROSCOPE. 



CHAPTER X. 

 MICROSCOPIC FORMS OF ANIMAL LIFE -.-PROTOZOA. 



391. PASSING-OX, now, to the Animal Kingdom, we begin by direct- 

 ing our attention of those minute and simple forms, which correspond in 

 the Animal series with the Protophyta in the Vegetable (Chap, vi.); and 

 this is the more desirable, since the formation of a distinct group to 

 which the name of PROTOZOA (first proposed by Siebold) may be appro- 

 priately given, is one of the most interesting results of Microscopic 

 inquiry. This group, which must be placed at the very base of the Animal 

 scale, beneath the great Sub-Kingdoms marked-out by Cuvier, is character- 

 ized by the extreme simplicity that prevails in the structure of the beings 

 composing it; the lowest of them being single protoplasmic particles or 

 ' jelly specks;' whilst even among the highest, however numerous their 

 units may be, the^e are (as among protophytes, 227) mere repetitions 

 of one another, each capable of maintaining an independent existence. 

 In this there is a very curious and significant parallelism to the earliest 

 embryonic stage of higher Animals. For the fertilized germ of any one 

 of these first shapes itself as a single cell; and then, by repeated binary 

 subdivisions, develops itself into a morula or ' mulberry-mass ' of cells 

 (Fig. 403), corresponding to the ' multicellular ' organisms met with 

 among the higher Protozoa (Fig. 350). There is, so far, in neither case, 

 any sign of that ' differentiation ' of organs which is characteristic of the 

 higher Animals; but whilst, in the Protozoon, each cell is not merely 

 similar to its fellows, but is independent of them, the morula, in such as 

 go on to a higher stage, becomes the subject of a series of developmen- 

 tal changes, tending to the production of a single whole, whose parts are 

 mutually-dependent. The first of these changes is its conversion into a 

 gastrula or primitive stomach, whose wall is formed of a double mem- 

 brane, the outer lamella, or ectoderm, being derived directly from the 

 external cell-layer of the morula, whilst the inner, or endoderm, is formed 

 by the * invagination ' of that layer into the space left void by the disso- 

 lution of the central cells of the ' morula.' 1 This gastrula- stage, as we 



1 It has not yet been certainly ascertained that the endoderm is formed by 

 invagination in all cases ; but as several of the supposed exceptions have dis- 

 appeared under the light of fuller investigation, it seems probable that the re- 

 mainder will be found conformable to the general rule. 

 1 



