THE VITAL PROPERTIES OF THE CELL 279 



cell membranes become dissolved. A connecting bridge, which 

 continually grows broader, develops ; after which the proto- 

 plasmic masses stream together from all sides, until the two in- 

 dividuals become transformed into a single large vesicle. The 

 two nuclei, each accompanied by a ceiitrosome, travel towards 

 each other, and place themselves in contact, but, according to 

 Ishikawa, do not fuse (VII. 25). After a time, the conjugating 

 pair of Noctilucw again divide into two cells, a partition membrane 

 having developed between them. At the commencement of this 

 division, the pair of nuclei, which have united together, become 

 extended ; they then become constricted in the middle, and divide 

 into two, after which they separate again, the result being that 

 each Noctiluca contains half of each nucleus. Thus the result 

 of conjugation is the production again of two individuals, each 

 of which possesses a nucleus of twofold origin. Fertilisation is 

 followed sooner or later by active multiplication by means of 

 budding and spore formation. 



The, ConjugatsB (VII. II) are of especial importance in the 

 study of primitive modes of fertilisation. This order is sub. 

 divided into three families : the Desmidiacese, the Mesocarpeas, and 

 the Zygnemaceae. 



Klebahn (VII. 27) has discovered the minute details of the 

 process of fertilisation in two species of Desmidiacea3 : the Clos 

 terium and Cosmarium. 



Two Closterium cells, which are shaped somewhat like bent 

 sickles, lie lengthwise against each other, being kept in contact 

 by a gelatinous secretion ; each then develops a protuberance near 

 its centre. The two protuberances come closely into contact and 

 fuse, whilst the wall separating them dissolves, to form a conju- 

 gation canal common to both. Here all the protoplasm from both 

 the conjugating Closterium cells gradually collects, and, detaching 

 itself from the old cell membrane, fuses to 

 form a single globular body, which finally 

 becomes surrounded by a membrane of its 

 own. 



This zygospore or zygote, which has been 

 produced by the fusion of two similar indi- 

 viduals, now passes through a resting stage, 

 which lasts for several months (Fig. 150). FIG. 150. Zygote of 

 It contains two nuclei, which were derived ciosterum, just before 



germination. (After Kle- 



from the two cells, and which remain apart bahn, PL XIIL, Fig. 3.) 



