THE VITAL PROPERTIES OF THE CELL 283 



very account are interesting, are seen in various species of 

 ZygnemacecB', they are worth noticing, for they teach us the way 

 in which sexual differences may at first develop. 



For instance, in Monjeotia, as in the Desmidiacete, the two proto- 

 plasmic bodies enter the conjugation canal and there fuse together 

 to form a zygote, which becomes globular, expresses fluid, and 

 surrounds itself with a membrane. In this case both cells 

 behave exactly alike ; neither can be termed male or female. 



In other species, such as Spirogyra (Fig. 153), one cell remains 

 passively in its membrane, and is sought out by the other, which 

 in consequence may be called the male. It wanders into the 

 conjugation canal, and, passing through it, reaches the female cell, 

 as though attracted by it; they then fuse to form a zygote (Fig. 

 153 A, a). When the zygote is treated with reagents and staining 

 solutions, it can be further established, that soon after the union 

 of the cells their nuclei approach each other, and unite to form the 

 germinal nucleus. ' Since in a thread all the cells act either as 

 males or females, one of the two conjugating threads generally 

 has all its cells empty, whilst the other contains a zygote in each 

 cavity (Fig. 153 B). The zygote surrounds itself with a separate 

 cell- wall, after which it generally rests until the next spring, 

 when it commences to germinate, and finally, by means of trans- 

 verse divisions, develops into a long Spirogyra thread. 



The above-mentioned distinction between male and female 

 Spirogyra threads by no means invariably occurs. For instance, 

 it may happen that a thread bends back on itself, so that one end 

 comes into the neighbourhood of the other. Under such condi- 

 tions, cells situated at the opposite ends of the same thread con- 

 jugate together, so that those which under other circumstances 

 would have functioned as male cells now play the part of female 

 cells. 



In the above-mentioned families of Noctilucse and Conjugate 

 and in others, such as Diatomaceas, Gregarinas, etc., the large pro- 

 toplasmic bodies are enclosed in membranes ; these pair, after 

 having passed through periods of vegetative multiplication by 

 simple division. A second series of primitive modes of sexual 

 reproduction is afforded us by lower plant organisms, such as 

 some of the Algae. For purposes of reproduction they develop 

 special cells, the swarm-spores, which are distinguished from the 

 vegetative cells by their small size, by the absence of a cell 

 membrane, and by the presence of two flagella or numerous cilia, 



