Hydrodtctyeae 



221 



smaller masses, until each portion contains only one nucleus. Reproduction 

 also occurs by isoplanogametes, which escape from the mother-ccenocyte by a 

 lateral pore. The gametes, which are smaller than the zoogonidia, are formed 

 in much greater numbers, and their escape is preceded by a swelling of the 

 inner layer of the coanocyte-wall. This layer ruptures the outer layers and 

 protrudes as a large vesicle in which the gametes swarm, after which they 

 conjugate in pairs, forming spherical zygotes. The latter rest for a short 

 period and then produce two or four large biciliated zoogonidia, which on 



Fig. 145. Hydrodictyoti reticulatum (L.) Lagerb. A, nat. size; B, small portion of a young 

 colony, x 110 ; C, part of a large crenocyte containing a very young colony, x 110 ; D, 

 polyhedral form assumed by the biciliated zoogonidia which arise on the germination of 

 the zygote ; E, young colony formed by the apposition of quiescent zoogonidia which arise 

 within the polyhedral cell. D and E, x about 300 (after Pringsheim). 



coming to rest assume a polyhedral form (fig. 145 D). The repeated division 

 of the contents of this polyhedral body results in the formation of numerous 

 zoogonidia which by apposition give rise to a new net-like coenobium 

 (fig. 145 E). 



Hydrodictyon reticulatum has a wide distribution in both the eastern and western 

 hemispheres, generally occurring in ponds and rivers. In the River Nile the young 

 coenobia float down the stream in myriads during the flood. The adult coenocytes are 

 4 5 mm . in length just before they become zoogonidangia. 



Klebs ('96) conducted experiments of much interest on Hydrodictyon reticulatum. He 

 showed that the transference of coenobia, which had been cultivated in a weak nutritive 

 solution under bright illumination, to pure water resulted in the formation of zoogonidia, 

 but in darkness the formation of zoogonidia could not be thus induced. In the case of 

 ccanobia cultivated in sugar solutions (maltose) and then transferred to pure water, they 



