. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. AXIMALS AND PLANTS. the cell itself; the vegetable cell being surrounded outside its pri- mordial utricle by a thick non-nitrogenous cuticle, the cellulose capsule; while the animal cell possesses a very delicate nitrogenous membrane, or instead of this only a more viscous bouiidary layer of of its own semi-fluid contents. Nevertheless, there arc also vegetable cells provided only with a simple naked primordial utricle; and, on the other hand, animal tissues which resemble vegetable tissues
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. AXIMALS AND PLANTS. the cell itself; the vegetable cell being surrounded outside its pri- mordial utricle by a thick non-nitrogenous cuticle, the cellulose capsule; while the animal cell possesses a very delicate nitrogenous membrane, or instead of this only a more viscous bouiidary layer of of its own semi-fluid contents. Nevertheless, there arc also vegetable cells provided only with a simple naked primordial utricle; and, on the other hand, animal tissues which resemble vegetable tissues in the fact that the cells remain independent and develop a capsule (chorda dorsalis, cartilage, supporting cells in the tentacles of hydroids, fig. 6). FIG. G.— a, Vegetable parenchyma (after Sachs). I, Axial-cells from the tentacles of Cam- panularia. Neither can we, as has been done by many investigators, regard the multicellular composition of the body as a necessary sign of animal life. For not only are there many unicellular algse and fungi, but also animal organisms which are composed of one simple or complexly differentiated cell (Protozoa). Finally, it is not possible to see any reason why unicellular animals should not exist, especially when we consider that the cell forms the starting-point for the development of the animal body. 3. Least of all can a test be found in the reproductive processes. In plants indeed we find a predominance of the asexual method of increase by spores and buds, but similar methods of increase are widely present amongst the lower and more simply organised ani- mals. Sexual reproduction is effected both in animals and plants by processes which are essentially similar; consisting in both of the fusion of the male element (sj)ermatozoon) with the female element (ovum) ; and the form of these elements presents in both kingdoms a great agreement, at any rate they are in every case derived from cells. The structure and position of the generativ
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