456 THE AMERICAN NATURALIST. [VOL. XXXVIII. 



consider two sets of factors. There is an evident contraction of 

 the protoplasm in many examples because water is given off. 

 The shrinkage of the surface would undoubtedly form furrows, 

 but, as Harper has pointed out, these furrows do not develop in 

 an accidental manner. Non-nucleated masses of protoplasm are 

 never separated from the nucleated, but the segmentation pro- 

 ceeds after a system by which the final products contain only 

 one nucleus or at most a limited number. So it is probable that 

 the nuclei are the ultimate centers controlling the segmentation 

 which at its commencement may be quite irregular. This 

 explanation of sporogenesis in the plasmodium and the spo- 

 rangium is not altogether satisfactory for the cell division of 

 Cladophora, the abstriction of conidia or the development of the 

 gametes of a mould. In these examples the cleavage begins at 

 definite regions of the plasma membrane, so that the stimulus 

 must be local, and the direction of the plane has a definite 

 relation to the axis of the plant. 



It is important to note (see Harper, : oo, p. 240-249) how 

 inadequate are some of the well-known theories of the segmen- 

 tation of protoplasm as explanations of cleavage by constriction. 

 Hofmeister's law ('67) that cell division is across the axis of 

 growth obviously cannot be applied to the irregular segmentation 

 in the plasmodium and sporangium, nor is Sachs' well-known law 

 of growth in vegetative points adequate. Sachs, '94, and in the 

 Lectures on the Physiology of Plants, chap. XXVII, conceives 

 a growing point of a higher plant or an embryonic structure as a 

 mass of protoplasm whose cell walls are determined by principles 

 of rectangular intersection of perpendicular planes. The outer 

 form of the structure determines the angles of periclines and 

 anticlines and the transversals conform to these. There is not 

 the slightest hint of such an order in the distribution of cleavage 

 planes in the multinucleate masses of protoplasm just described 

 and Sachs' law in so far fails of general application whether or 

 not it be satisfactory for the conditions with which he especially 

 deals. There are also explanations of cell division, applicable to 

 the tissues of many higher organisms, based on the position of 

 the nuclear figure in the cell, which determines the position of 

 the cell plate but these theories cannot handle the events in the 



