THE ALVEOLAR LAYER AND CELL MEMBRANES 239 



tions can be similarly explained, i.e. that a primitive alveolar layer 

 becomes a firm membrane as the result of chemical alteration, 

 which leads as a consequence to the development of a new 

 alveolar layer beneath it, since according to the already described 

 laws governing foam, the layer of alveoli bordering on a rigid 

 membrane must always assume the character of an alveolar layer. 

 In Ley dig's work (1885) I can only definitely assert the radially 

 striated border of the epithelial cells of the lingual glands of 

 Pelobates to be a true alveolar layer. The outer radial zone of 

 the epithelial cells of Salamandra maculosa is, on the other hand, 

 much too thic,k to be regarded as an alveolar layer. This is also 

 obvious from the later description of these cells by Tangl 

 (1887). Pfitzner (1885) has also observed this border. It is 

 probably a case of the meshes of the whole of this zone being 

 radially arranged, by means of which the alveolar layer of 

 course becomes more indistinct. Carnoy (1884) has quite 

 certainly observed the alveolar layer, especially in the cells of 

 the intestine of Asellus, but his description is rather obscure. 

 The so-called membrane also, which he figures and describes 

 (though without structure) in the testicular cells of Lithobius, with- 

 out doubt belongs here. Now Carnoy considers the above- 

 described layer at the surface of the cells as a cell membrane, 

 and holds the view generally that all cells possess such a mem- 

 brane. I think, therefore, that he must have observed frequently 

 similar structures, and has regarded all these structures as cell 

 membranes. As a matter of fact he has later (1885) also 

 observed in the amoeboid motile testicular cells of insects such a 

 membrane, which was said to show a reticular structure, and to be 

 occasionally raised up in places. Finally, in 1886, he described 

 in the segmentation of Nematode eggs a so-called cell plate, 

 which, travelling from without inwards, was found later on in the 

 plane of separation of the two blastomeres, and within which 

 the splitting of the cells then takes place. I am inclined to 

 think that Carnoy is wrong when he assumes that the segmenta- 

 tion of the Nematode eggs goes on in two ways, i.e. first by 

 means of constriction, and secondly, by splitting in the interior 

 of this cell plate. It seems to me rather that he has overlooked 

 the peculiar process of the so-called flattening of the seg- 

 mentation cells after their constriction, and has regarded 

 the closely appressed cells as stages of division by splitting. 

 This alleged cell plate, however, which can hardly have anything 

 to do with what is usually termed a cell plate, is a double 

 alveolar layer in structure, just such as I have also observed 

 at the limit of the appressed blastomeres of the eggs of the sea- 

 urchin. Carnoy derives later from the two separated layers of 



