572 THE AMERICAN NATURALIST. [VOL. XXXVIII. 



being accompanied by a pair of cilia, or those of CEdogonium 

 whose colorless forward end bears a crown of numerous cilia. 

 The zoospore stands among the higher forms for a type of motile 

 organism that is very close to the bottom of the assemblage of 

 groups and developmental lines which make up the Algae. The 

 forms most closely related to the zoospore are in the family 

 Chlamydomonadeae of the Volvocales. But at this general 

 low level of the plant kingdom there are several groups whose 

 members pass most of their .lives in motile conditions (Volvo- 

 cales, Flagellates and Peridinales) and the cells of all of these 

 types resemble zoospores to a greater or less degree in their 

 structure and habits, so that this condition represents a wide- 

 spread and well defined stage of evolutionary development. 

 Therefore when zoospores are formed in the life history of some 

 higher plant they represent a return on the part of the organism 

 for a short time to the structure and mode of life of an ancestry 

 perhaps related in some way to the groups that still have the 

 motile habits throughout most of their existence. 



For these reasons close comparisons in structure between the 

 zoospore and motile Algae will be interesting and should help to 

 explain the peculiarities of these cells. These peculiarities 

 chiefly concern the organ that forms the cilia (blepharoplast), 

 which becomes very complex in the sperm, and the pigment 

 spot. 



Unfortunately studies upon these problems have been few and 

 we are not prepared to make a general statement of the condi- 

 tions. The most recent investigation on the structure of the 

 zoospore is that of Timberlake (: 02), but Strasburger has written 

 extensively on the subject, especially in the Histologische Bei- 

 trage ('92 and : oo). The later paper (:oo, p. 177-215) reviews 

 the entire subject of cilia formation. Dangeard has presented 

 an account of the Chlamydomonadeae, '99, and in :oi described 

 especially Polytoma, comparing its structure with that of the 

 animal spermatozoan. 



Polytoma (see Fig. 9 a) is a colorless organism but its cell 

 structure and life history place it unquestionably among the 

 Chlamydomonadeae. The two cilia arise from a small body 

 (blepharoplast) situated at the extremity of the cell. A delicate 



