274 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



plants, where almost every careful investigation has dis- 

 covered a new type. Widely divergent types have been 

 shown to occur in different tissues in the same individual 

 (Nemec, 1898; Strasburger, 1900, p. 119, also, p. ii2,_/".) 

 or even in the same tissue (Davis, 1901 : Hertwig, 1898). 

 In view of this we may conclude that the process of spindle 

 formation is a plastic one, easily modified in the course of 

 phylogeny and even of ontogeny, and that its study deserves 

 to be seriously taken up as a matter of vital importance to 

 an understanding of cell mechanics. 



SUMMARY. 



The first spindle formation of the pollen-mother-cells of 

 Agave amei'icana L. presents the following features of 

 especial interest. 



1. During the early stages of the spindle it is enclosed 

 in a special membrane (of cytoplasmic origin) which forms 

 a complete investment around it, and is called the spindle 

 wall; the functions of this membrane appear to be compa- 

 rable to those of the nuclear wall and the limiting membrane 

 of the cytoplasm ; it appears to be a unique structure. 



2. Unlike most cases hitherto described, no weft stage is 

 present; this is probably due to the presence of the spindle 

 wall, which prevents the expansion of the nucleus, and con- 

 sequently the formation of the weft. 



3. The genetic {i. e., spindle-forming) fibres are radial 

 from the beginning, and are attached to both the nuclear 

 and the spindle wall. 



The second spindle formation differs in toto from the 

 first and may be summarized as follows : The genetic 

 (spindle forming) fibres form in close contact with the 

 nuclear wall, take on a radial arrangement, extend outward 

 into the cytoplasm, assemble in groups, and form cones 

 which by their fusion give rise to the spindle. The process 

 resembles in general the spindle formation of the spore- 

 mother-cells of Equisetuni. 



