Zoogonidia 133 



notable exception in the Conjugates, in which motile reproductive cells are 

 entirely wanting. They are also absent in the family Caulerpacese of the 

 Siphonales and in the Autosporacea? of the Protococcales. They arise in 

 a zoogonidangium, which may be formed from an ordinary vegetative cell 

 without change of form, or may be a considerably modified asexual repro- 

 ductive organ. 



The zoogonidia are small protoplasts formed singly by the rejuvenescence 

 of the contents of a cell, or more frequently in numbers either by successive 

 bipartition of the original protoplast or by free cell-formation 1 . The whole 

 of the original protoplast, or in the case of coenocytes the entire protoplasm, 

 is not necessarily used up during the formation of the zoogonidia. In many 

 cases a peripheral hyaline part of the protoplasm takes no part in this 

 formation of reproductive cells, but becomes converted into a colourless 

 mucilage which plays an important part in the escape of the zoogonidia. 

 It is often largely owing to the pressure exerted by the swelling of this 

 mucus that the weakest spot in the wall of the zoogonidangium is broken 

 through and the zoogonidia (or ' swarm-spores ') permitted to escape. These 

 may swim away directly into the surrounding water or they may be held for 

 a limited time within a delicate vesicle protruded from the orifice of the 

 zoogonidangium. 



It is possible to induce the formation of zoogonidia in some Chlorophycese by trans- 

 ferring them from a nutritive solution to distilled wa,ter or pure spring water, or to a 

 solution of much less concentration. Placing in the dark sometimes assists the process, 

 as in the case of Vaucheria. Change from a lower to a higher temperature (such as when 

 Green Algee are brought from their natural habitats into a laboratory) may also cause the 

 production of zoogonidia. 



The zoogonidia are naked protoplasts destitute of a cell-wall, and 

 furnished with two 2 , four, or many cilia attached to the narrower anterior 

 end ; that is, the end which is carried foremost in swimming. This anterior- 

 extremity is sometimes drawn out into a small beak-like or wart-like 

 projection of a hyaline character, in which case the cilia are attached at 

 or around its base, as in the zoogonidia of Cladophora (fig. 88.4) or of 

 (Edogonium (fig. 88 D}. This beak-like part of the protoplast is stated by 

 Strasburger to consist of ' kinoplasm.' Each zoogonidium may possess one 

 or more chloroplasts, and is frequently provided with two minute contractile 

 vacuoles situated in front of the nucleus close to the anterior extremity. 

 A red pigment-spot (or stigma) is often present, usually in an anterior and 



1 In the genus Vaucheria the single large zoogonidium is a compound structure which may 

 in all probability be regarded as representing the fusion of a large number of small biciliated 

 zoogonidia such as are typical of the Isokontee. 



2 The zoogonidia of certain Algte belonging to the Heterokontas have been described as 

 possessing only one cilium, but in those cases in which the zoogonidia have been carefully 

 re-examined the second, much shorter, cilium has been discovered. 



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