46 INTRODUCTION. 



At the present writing it is the opinion of the author that individu- 

 alized centrosomes or centrospheres do not occur in plants above the 

 liverworts, and they are certainly absent in certain species of these 

 (Anthoceros). On the whole, these structures are well established in 

 only a few Thallophyta. 



As the writer has already stated in a former paper (Mottier, 1900), 

 if we take into consideration only such plants as Fucus, Stypocaulon, 

 Dictyota, and certain Ascomycetes, there are good grounds for the 

 view that the centrosome is an organ of morphological value ; but the 

 evidence furnished by these forms, however convincing it may seem, 

 is not quite sufficient, especially in the light of our knowledge of kary- 

 okinesis in forms in which centrosomes or centrospheres have not been 

 found ; for there is no reason for believing that the spindle fibers in 

 plants devoid of centrosomes are of a different substance from the 

 radiations or spindle fibers developed in connection with an aster. 



Space will not permit of a discussion of such questions as whether 

 the radiations are outgrowths of the centrosome considered as a mor- 

 phological unit, or constructed out of the kinoplasm by the centrosome, 

 or whether the centrosome is only a denser mass of kinoplasm, formed 

 by the meeting of the polar radiations, and which may persist after the 

 radiations and spindle fibers have disappeared. It may be stated in 

 this connection that in plants there is little to support the view that the 

 radiations are centripetal or centrifugal currents. They do not seem 

 to be currents at all. We understand radiations and spindle fibers to 

 be fine, more or less homogeneous, kinoplasmic threads which are 

 capable of contracting, extending, or becoming changed into a uniform 

 and homogeneous mass. 



We have now to consider the relation of the centrosome to the 

 blepharoplast, or cilia-bearer, which is so well known in the sperma- 

 tozoid of the Archegoniates (see Chapter V). 



Belajeff, Ikeno, and Hirase and a few others regard the blepharo- 

 plast of the fern and certain gymnosperms as the homolog of the centro- 

 some. It seems to the author that such a conclusion is merely a hasty 

 judgment, which does violence to the facts as they are known at present. 

 The development and function of the blepharoplast, as will be seen 

 from the chapter referred to, shows clearly that this structure lacks the 

 more essential distinguishing characteristics of the normal centrosphere, 

 as it is known in the cases most thoroughly investigated. The bleph- 

 aroplast is not the center of kinoplasmic radiations which form a 

 karyokinetic spindle. So far as has been shown the radiations of the 

 blepharoplast primordia take no part in the formation of the spindle. 

 These primordia do not divide to give rise to new blepharoplasts, but 



