CLA.SSIFICATI01S- OF THE ANIMAL KINaDOM. 205 



On the whole, while I hesitate to absolutely identify the endo- 

 plast of an Infusorian with the nucleus of a histological cell, and 

 can find no analogue for the endoplastula in the latter, I think 

 that Von Siebold's view holds good, and that the higher Infusoria 

 are unicellular animals, in the sense that Mucor, Vaucheria, and 

 Gaulerpa are unicellular plants. 



Nevertheless it must be admitted, on the other hand, that 

 though the view for which Ehrenberg has so long contended, that 

 the Infusoria possess, in miniature, an organization, in a broad 

 sense, as complex as that of the higher animals, is not tenable, 

 the great majority of them are far more highly organized than was 

 suspected before that indefatigable observer commenced his long 

 and remarkable series of investigations. 



II. The Metazoa. 



The germ undergoes differentiation into histogenetic cells ; and 

 these cells become arranged into two sets, the one constituting 

 the outer wall of the body, while the other lies internal to the 

 foregoing, and forms the lining of the alimentary cavity, when, 

 as is usually the case, a distinct alimentary cavity exists. In 

 the embryo, the representatives of these two layers are the 

 epihlast and hypoblast. In the adult, they are the ectoderm and 

 the endoderm, which answer to the epidermis, and the epithelium 

 of the alimentary canal, in the higher animals. 



All the Metazoa, in fact, commence their existence in the 

 form of an ovum, which is essentially a nucleated ceU, supple- 

 mented by more or less nutritive material, ovfood-yelJc. The ovum, 

 after impregnation, divides into blastomeres, giving rise to a 

 Morula (Hasckel), in the midst of which arises a cavity, the ilasto- 

 coele {cleavage- cavity, ^' Ftirchungsholile" of the Grermans), which 

 may be larger or smaller, filled only with fluid, or occupied by 

 food-yelk. "When it is largest, the blastomeres, united into a 

 single layer, form a spheroidal vesicle, enclosing a correspondingly 

 shaped blastocoele. When it is reduced to a minimum, the 

 Morula is an almost solid aggregation of blastomeres, which may 

 be nearly equal in size, or some much larger than others, in conse- 

 quence of having undergone less rapid division. The next stage 

 in the development of the embiyo of a Metazoon consists (in all 

 cases except a few parasitic anenterous forms) in the conversion 

 of the Morula into abody ha^dng a digestive cavity, or a Gastrula. 



