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CHAPTER XII 



MICROSCOPIC FORMS OF ANIMAL LIFE PROTOZOA 



PASSING on, now, to the Animal Kingdom, we begin by directing 

 our attention to those minute and simple forms which correspond in 

 the animal series with the Protophyta in the vegetable (Chap. VIII.) ;. 

 and this is the more desirable since the formation of a distinct group 

 to which the name of PROTOZOA (first proposed in this sense by 

 Siebold) may be appropriately 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, is characterised by the apparent 

 simplicity that prevails in the structure of the beings that compose 

 it, the lowest of them being single protoplasmic particles or * jelly- 

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

 units may be, these are (as among protophytes) 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 fertilised 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, corresponding to the * multicellular ' organisms met with 

 among the higher Protozoa. 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 

 developmental 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 membrane, the outer lamella, or ectoderm, 1 

 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 dissolution of the central 

 cells of the ' morula.' This gastrula-stage? as we shall see hereafter, 

 remains permanent in the great group of Ccelentera, though the 

 endoderm and ectoderm are separated from each other in its higher 

 forms by the development of generative and other organs between 



1 The terms epiblast and hypoblast are generally used by English embryologists 

 in place of the ' ectoderm ' and ' endoderm ' used here. 



2 The gastrula- stage is in a number of cases brought about by a concentric split- 

 ting of the walls of the morula into two layers, and by the appearance at one point 

 of an orifice which leads into the central cavity ; this cavity is the original segmenta- 

 tion cavity of the morula, and not a fresh cavity, as in ' invaginate gastrulse.' 



