210 BIOLOGY OF THE PROTOZOA 



the continued life of many of the higher plants, and for the con- 

 tinued life of the tissue cell cultures in the hands of Carrel and 

 others (see Chapter VII). 



In many Protozoa there is unmistakable evidence of such reorgan- 

 ization processes which will be described in the following pages; 

 in many there is no visible evidence, but in such cases and in the 

 absence of other possibilities of reorganization, it is permissible to 

 assume that reorganization processes which escape the most vigilant 

 watchfulness of the observer, do actually occur. 



A. Division in Mastigophora. — With very few exceptions cell 

 division in flagellates is longitudinal, beginning as a rule at the 

 anterior or flagellar end, the cleavage plane passing down through 

 the middle of the body. As the halves separate the two daughter 

 cells usually come to lie in one plane, so that final division appears 

 to be transverse. In the majority of forms the individuals divide 

 while freely motile, but this is by no means universal, variations 

 in this respect occurring in the same family and even in the same 

 genus. 



As there are few details in the structure of a simple flagellate on 

 which to focus attention, descriptions of division processes are 

 practically limited to the history of the nucleus, kinetic elements 

 and the more conspicuous plastids. Here, in the main, are fairly 

 prominent granules of different kinds which divide as granules, and, 

 save for the chromatin elements of the nucleus, without obvious 

 mechanisms. 



In the simpler cases there is little evidence that can be interpreted 

 as reorganization at the time of division, and the little we find is 

 limited to the motile organs. In the more complex forms, however, 

 there is marked evidence of deep-seated changes going on m the cell. 



The earlier accounts of cell division in the simpler flagellates 

 described an equal division of all parts of the body including longi- 

 tudinal division of the flagellum, if there were but one, or equal dis- 

 tribution if there were two. One by one such accounts have been 

 checked up by use of modern technical methods until today there 

 is very little substantial evidence of the actual division of a flagel- 

 lum. The basal body and the blepharoplast usually divide, but 

 the flagellum either passes unchanged to one of the daughter cells 

 as in Crithidia, Trypanosoma, etc., or is absorbed in the cell. In 

 some doubtful cases it may be thrown off. If the old flagellum is 

 retained in uniflagellate forms the second flagellum develops by 

 outgrowth from the basal body or the blepharoplast. If the old 

 flagellum is absorbed, both halves of the divided kinetic element 

 give rise to flagella by outgrowths (Fig. 49, p. 95). Similarly, 

 if there are two or more flagella, one or more may be retained by 

 each daughter cell while the other, or full number, is regenerated. 

 In some cases, as in Herpetomonas musca-domesticae, the regenera- 



