POLARITY OF THE CELL 55 



protoplasm produced by plasmolysis are likewise secretion-products, 

 and that the secondary thickening of plant-membranes is produced 

 in the same way. These facts, together with the scanty available 

 zoological data, indicate that the formation of membranes by secre- 

 tion is the more usual and typical process. 1 



The chemical composition of the membrane or intercellular sub- 

 stance varies extremely. In plants the membrane consists of a basis 

 of cellulose, a carbohydrate having the formula C 6 H 10 O 5 ; but this sub- 

 stance is very frequently impregnated with other substances, such 

 as silica, lignin, and a great variety of others. In animals the inter- 

 cellular substances show a still greater diversity. Many of them are 

 nitrogenous bodies, such as keratin, chitin, elastin, gelatin, and the 

 like ; but inorganic deposits, such as silica and carbonate of lime, are 

 common. 



H. POLARITY OF THE CELL 



In a large number of cases the cell exhibits a definite polarity, its 

 parts being symmetrically grouped with reference to an ideal organic 

 axis passing from pole to pole. No definite criterion for the identi- 

 fication of the cell-axis has, however, yet been determined ; for the 

 general conception of cell-polarity has been developed in two differ- 

 ent directions, one of which starts from purely morphological con- 

 siderations, the other from physiological, and a parallelism between 

 them has not thus far been fully made out. 



On the one hand, Van Beneden ('83) conceived cell-polarity as a 

 primary morphological attribute of the cell, the organic axis being 

 identified as a line drawn through the centre of the nucleus and the 

 centrosome (Fig. 22, A). With this view Rabl's theory ('85) of 

 nuclear polarity harmonizes, for the chromosome-loops converge 

 toward the centrosome, and the nuclear axis coincides with the cell- 

 axis. Moreover, it identifies the polarity of the egg, which is so 

 important a factor in development, with that of the tissue-cells ; for 

 the egg-centrosome almost invariably appears at or near one pole of 

 the ovum. 



Heidenhain ('94, '95) has recently developed this conception of 

 polarity in a very elaborate manner, maintaining that all the struc- 

 tures of the cell have a definite relation to the primary axis, and that 

 this relation is determined by conditions of tension in the astral rays 



1 Strasburger ('97, 3, '98) believes membrane-formation in general to be especially con- 

 nected with the activity of the " kinoplasm," or filar plasm of which he considers the " Haut- 

 schicht," as well as the spindle-fibres, to be largely composed. In support of this may be 

 mentioned, besides the mode of formation of the partition-walls in the division of plant- 

 cells, Harper's ('97) very interesting observations on the formation of the ascospores in 

 Erysiphe (Fig. 33), where the spore-membrane appears to arise directly from the astral 

 rays. 



