HISTOLOGY OF THE FROG 79 



as being nearly synonymous, the former having a slightly 

 wider and more general meaning than the latter. 



The ultimate structure of protoplasm, as we have said, is 

 a matter hotly debated at the present time, and it would be 

 out of place to enter here into a discussion of the different 

 views of different observers. All are agreed on one thing, that 

 "protoplasm" is not a chemical compound of definite 

 formula, but a mixture of several different kinds of albumin- 

 ous bodies, each of great molecular complexity. Speaking 

 generally, it may be said that protoplasm is a mesh-work, or 

 rather a sponge-work, of a denser albuminous substance, in 

 the spaces of which a more fluid substance is contained. The 

 granules so generally present are of various kinds, and may be 

 fatty, albuminous, or starchy; some of them are of complex 

 constitution, and have been described as behaving like a 

 miniature cell within the cell. The nucleus is described as 

 having a surrounding membrane composed of a substance 

 called amphipyrenin, the chemical distinctness of which is, 

 however, very doubtful. In the nucleus there is a network 

 of fibrils, composed of a substance called linin, which does 

 not stain with the usual dyes ; and there is a network of 

 coarser fibrils, composed of a deeply-staining substance 

 called chromatin or nuclein. We know something of the 

 chemical constitution of nuclein ; it is a compound of a 

 complex organic acid nucleic acid, rich in phosphorus, with 

 albumen. Linin is probably an allied body, less rich in 

 phosphorus, and the amphipyrenin is probably identical with 

 linin. Other substances have been described in the nucleus, 

 but they need not detain us. The cell, then, is a very 

 complicated body, and it has an organisation, a complex 

 constitution, such as Dujardin postulated for his sarcode. 

 It consists essentially of a cell-body or cytoplasm and a 

 nucleus, and both cytoplasm and nucleus are mixtures of 

 several very complex chemical compounds whose formulae 

 have not yet been worked out. 



It is clear that both kinds of corpuscles contained in the 

 frog's blood, the haematids and the leucocytes, are cells. Both 

 have a cell-body and a nucleus, yet they differ in a marked 

 degree from one another. The leucocyte is comparatively 

 formless, and its characters are such as to afford an excellent 

 illustration of what we mean when we speak of an animal cell. 



