46 GEXEK.U. SKETCH OE THE CELL 



of the livin;;- i)bjcct. owiiii;" to differences of staining capacity. In 

 these egi;s the microsomes, thickly scattered through the alveolar 

 walls, stain deeply (Figs, ii, 12), while the alveolar spheres hardly 

 stain at all. When, therefore, the stained sections are cleared in 

 balsam, the contours of the alveolar spheres almost disappear, and the 

 eye is caught by the walls, which give at fir.st sight quite the appear- 

 ance of a granular reticulum, as it has been in fact described by many 

 observers. Careful study of the sections shows, however, that the form 

 and arrangeviciit of all the clcinents is almost identically the same as 

 in life. 



This result shows that careful treatment by reagents in some cases 

 at least gives a very faithful picture of the normal structure; and 

 while it should never be forgotten that in sections we are viewing 

 coagulated material, much of which is liquid or semi-liquid in life, we 

 should not adopt too pessimistic a view of the results based on fixed 

 material, as I think some of the experimenters referred to above have 

 done. Wherever possible, the structures observed in sections should 

 be compared with those in the living material. When this is imprac- 

 ticable we must rely on indirect evidence ; but this is in many cases 

 hardly less convincing than the direct. 



It is a very interesting and important question whether living 

 protoplasm that appears to the eye to be homogeneous does not really 

 possess a structure that is invisible, owing to the extreme tenuity of 

 the fibrillae or alveolar walls (as was long since suggested by Heitz- 

 mann and Butschli),^ or to uniformity of refractive index in the 

 structural elements. It is highly probable that such is often the case ; 

 indeed, Biitschli has shown that such " homogeneous " protoplasm in 

 Protozoa may show a typical alveolar structure after fixation and 

 staining. This explanation will not, however, apply to the young 

 echinoderm eggs (already referred to at p. 28), where the genesis of 

 the alveolar structure may be followed step by step in the living cell. 

 The protoplasm here appears at first almost like glass, showing at 

 most a sparse and fine granulation ; but after fixing and staining it 

 appears as a mass of fine, closely crowded granules. This may indi- 

 cate the existence of an extremely fine alveolar structure in life ; but 

 on the whole I beheve that these granules are for the most part coagu- 

 lation-products, since they cannot be demonstrated by staining intra 

 vitam, and they very closely resemble the coagulation-granules found 

 in structureless proteids like egg-albumin after treatment by the same 

 reagents. In common with many other investigators, therefore, I [ 

 believe that protoplasm may in fact be homogeneous dozvn to the\ 

 present lijnits of microscopieal vision. 



One of the must beautiful forms of cyto-reticulum with which I 



1 Cf. Ijiitschli, '92, 2, p. 169. 



