Lecture XL 89 



At the same time as the ejection of the sperms the solution of 

 the wall covering the beak of the oogonium is completed and the 

 sperms seem detained at the opening in the wall of the oogonium 

 so formed. Often several of them enter. One only, however, 

 fuses with the ovum, penetrating at the receptive spot. Its 

 cytoplasm merges with the cytoplasm of the ovum. In stained 

 specimens the passage of the nucleus of the sperm across the 

 cytoplasm of the ovum and its ultimate fusion with the nucleus of 

 the latter have been observed. Thus fertilisation is effected and 

 the oosperm, or cell formed by this union of the ovum and sperm, 

 contracts in the oogonium and secretes round itself a comparatively 

 thick cell-wall. In this condition it undergoes a period of rest. 

 When germination occurs the oosperm grows directly into a new 

 plant. 



These two methods of reproduction in Vaucheria are evidently 

 suited for different conditions. Thus the zoospore germinates 

 immediately after its cilia are cast off and its cell-wall is formed, 

 while the oosperm remains dormant for a period. The latter is 

 able to resist considerable desiccation, and consequently can sur- 

 vive unfavourable conditions. Further, it has been found that the 

 two processes are evoked by different stimuli : zoospores are 

 formed in response to increase of light or a scarcity of food ; 

 sexual reproduction is initiated in -response to a supply of sugar, 

 a high temperature and a bright light. 



In Volvox we observed a differentiation into reproductive and 

 somatic cells ; also among the latter category, specialisation was 

 noticeable. The somatic cells round the anterior pole are 

 furnished with stigmata and are* apparently capable of perceiving 

 light, those round the posterior pole are not furnished with these 

 organs. The differentiation of Vaucheria is quite as marked, 

 but of another kind. Certain branches of the ccenocyte are 

 furnished with chloroplasts and these function as organs of photo- 

 synthesis ; others, the rhizoids, are colourless and to them the 

 function of anchoring the ccenocyte is assigned. Thus in both 

 cases structural differentiation and division of labour are associated. 



PRACTICAL WORK. 



Detach a portion of Vaucheria from the felt or mat of fibres covering 

 moist soil, causing as little injury as possible to the filaments. Mount it in 

 water and cover. 



After examination of the plant, make drawings of three optical sections, 

 one in a plane immediately within the cell-wall, where it is nearest the 

 observer, one in a median plane and another in a plane in the protoplasm at 



