718 THE AMERICAN NATURALIST. [VOL. XXXIX. 



tophytes). They sometimes approach the centrosphere very 

 closely in their morphology. 



The third and highest type of spindle formation in plants is 

 that illustrated in the mitoses within the spore mother-cell which 

 were given special treatment in Section III (Amer. Nat., vol. 38, 

 p. 725, October, 1904). In this remarkable cell the spindles 

 develop from a mesh of independent fibrillae which at prophase 

 more or less completely surround the nucleus. The poles of 

 the spindle arise by the grouping of cones of fibrillae so that a 

 single axis is finally established but without any kinoplasmic cen- 

 ters at the poles. This type of spindle formation which may be 

 termed the free fibrillar type is one of the most interesting 

 cytological peculiarities of plants. It has been found in all 

 types whose sporophytic phase terminates its history with a 

 spore mother-cell, although the accounts in the Hepaticae are 

 not in full accord. 



Is it possible to connect the various types of spindle formation 

 with one another and to establish any evolutionary tendencies in 

 the processes involved ; and have the different manifestations of 

 kinoplasm such as centrosomes, centrospheres, polar caps, free 

 fibrillar condition, and the mysterious structure called the ble- 

 pharoplast any genetic relation to one another ? The confusion 

 is so great among the thallophytes that the author sees little 

 hope at present of establishing clearly any relationships between 

 the types of centrospheres and centrosomes with their systems 

 of radiations (asters) and we must patiently wait for more infor- 

 mation. And respecting the origin of these structures from the 

 simpler types of mitosis we are absolutely in the dark. But 

 the relation which polar caps and the free fibrillar type of spin- 

 dle formation bear to centrospheres is less perplexing and it 

 seems possible to define certain common features among these 

 structures which hold them together with a degree of unity in 

 their relations to mitosis. That phase of the subject will be 

 considered in this treatment. The Hepaticae as a group occupy 

 an interesting position with respect to the character of mitotic 

 phenomena at various periods of ontogeny, between conditions 

 in the pteridophytes, which are obviously similar to the sperma- 

 tophytes, and conditions in the thallophytes. This was brought 



