t CYTOPLASM ■ 3 



In optical section these films or lamellae surrounding the droplets are 

 easily interpreted wrongly as a reticulum. This theory is supported by 

 the behaviour of the artificial emulsions made by Biitschli, which exhibit 

 many striking resemblances to cytoplasm. 



Granules or fibrillae (chondriosomes, chromidia, etc., see Chapter VL) 

 may be suspended at the nodes of the apparent meshwork formed by the 

 denser fluid. Thus this theory is not incompatible with the fibrillar 

 theor5^ the alveolar structure applying to the matrix in which the fibrillae 

 are embedded. 



The alveolar theory is the one that fits in best ^^dth the known 

 properties of the cytoplasm, and especially with its undoubtedly fluid 

 nature. For living cytoplasm is, physically, a fluid — often very viscid 

 indeed, but nevertheless fluid. During life streaming movements are 

 often observable in it. Moreover, bodies such as the nucleus, which are 

 very large relatively to the meshes or alveoli, can move through the 

 living cytoplasm. Examples of rapid change of position of the nucleus 

 in the cell are afforded by living Protozoa such as Amoeba, and bj^ the 

 movements of the pronuclei in fertilization in the Metazoa (Chapter III.). 

 Many other proofs of the fluid nature of the cytoplasm could be cited, 

 such as the spherical shape assumed by vacuoles and by fragments of 

 cytoplasm extruded from a cell into water. These facts are impossible 

 to reconcile with the presence of a permanent supporting reticulum 

 forming part of the essential structure of the cytoplasm. In many 

 kinds of cells supporting reticula and fibrillae are indeed undoubtedly 

 present, but these are of a different order of structure and belong to the 

 architecture of the cell as a whole, and not to the structure of the cyto- 

 plasm, which lies within the meshes of the supporting framework. 



(2) The Cell Membrane 



With few exceptions (certain Protozoa, leucocytes) animal cells are 

 plainly delimited by an outer metamorphosed layer of cytoplasm, or by 

 a membrane secreted by the cytoplasm, the distinction between the two 

 being often very difficult to make and indeed unreal. 



Sometimes, however, nuclear division is not followed by cell division 

 with development of a membrane between the two cells, and in this case 

 there arises a structure known as a syncytium, in which a number of 

 nuclei are embedded in a continuous mass of cytoplasm. Well-known 

 examples of syncytia are the plasmodia of Mycetozoa and the ectoderm 

 of Nematodes. 



In certain cells the membrane attains a much greater importance, 

 and may lose its connection with the underlying cytoi)lasm which 

 secreted it, so that the latter comes to lie more or less freely within it 

 as if in a box with which it had no close organic connection. In such a 



