io6 BOTANICAL GAZETTE [august 



shown to be appearances due to the intersection of the very 

 numerous astral radiations at a common point or region. The 

 achromatic figure is derived entirely by the rearrangement of the 

 cytoplasmic network. 



As the centrosome becomes more widely known it becomes 

 increasingly difficult to formulate for it any adequate definition. 

 There is scarcely a single attribute common to all true centrosomes; 

 nevertheless there are in general certain features which are fairly 

 characteristic of them as they appear in plants and animals, most 

 prominent among which are the position at the spindle poles with 

 all that this implies, the possession of an aster, and the division to 

 form daughter centrosomes. Because of many exceptions no one 

 of these by itself will definitely determine the morphological nature 

 of a structure possessing it, but when all of them are present we can 

 no longer doubt that we are dealing with a true centrosome. 



In a survey of the cilia-bearing structures of bryophytes, pterido- 

 phytes, and gymnosperms it is seen that in general the centrosome- 

 like characteristics of the blepharoplast become less and less evident 

 in passing upward through these groups, while the phenomena 

 connected with the bearing of cilia become increasingly prominent. 

 In the bryophytes the conflicting accounts leave us in some doubt 

 concerning the early history of the blepharoplast, but in some cases 

 at least it appears that centrosomes exist through several cell 

 generations and after the last mitosis function as blepharoplasts. 

 In those forms which show them only during the last division they 

 occupy the spindle poles and behave as typical centrosomes. In 

 the spermatids each simply elongates and bears two cilia. In the 

 Filicales, as shown by Yamanouchi's work on Nephrodium, the 

 blepharoplast is limited to the last mitosis and does not exhibit the 

 characters of a centrosome, having no division, no radiations, and 

 only occasionally occupying the pole of the spindle. It elongates 

 in intimate union with the spermatid nucleus and bears many cilia. 

 In the gymnosperms the blepharoplast, although surrounded by 

 prominent radiations, appears to play little or no active part in 

 mitosis. In its subsequent behavior it differs widely from the 

 blepharoplasts of the bryophytes and Filicales. After enlarging 

 it becomes vacuolate and breaks up into many fragments, which 



