REJUVENESCENCE IN NATURE. 273 



close to the cell-wall, against wKich they are firmly applied 

 during their completion. — This is the character of the 

 formation of the germ-cells of Valonia described by 

 Nageli. In the long tubes, when full-grown measuring 

 5 to li inch in length, of this unicellular marine Alga, 

 the organic contents form a thin investment to the wall, 

 while the large cavity is filled with salt water.* The 

 mucilaginous layer is bfeset with reticularly arranged 

 or scattered chlorophyll-vesicles. At particular spots, 

 especially at the bottom, more rarely in the apex of the 

 cell, are formed the germ-cells, sometimes singly, some- 

 times connected in groups. According to Nageli, they 

 originate as minute colourless globules, which acquire as 

 they grow an evident membrane and green granular 

 contents, chlorophyll-vesicles being formed in their in- 

 terior. The fully-developed germ-cells adhere as flattened 

 disks to the wall of the cell. Those which occur at the 

 upper end of the mother-cell usually germinate in the 

 still surviving mother-cell, breaking through its mem- 

 brane, probably locally softened, stopping up the orifices 

 again with their bases. The peculiar ramification of 

 Valonia arises from this growth of the young individuals 

 out of the old one. The germ-cells at the lower part of 

 the tube, remaining undeveloped, probably only become 

 free through the decay of the parent individual, to be- 

 come developed as separate individuals forming new 

 family-stocks. The case here mentioned is distinguished 

 from the mode of cell-formation minutely described in 

 Hydrodictyon, under D. 2, by the circumstance that only 

 particular points, and not the whole of the parietal 

 mucilaginous layer, passes into new cell-formation ; more- 

 over that the new cells exhibit a gradual increase in size 

 in the surviving mother-cell, while in Hydrodictyon they 

 attain at their first production the full size which they 

 are destined to acquire inside the mother-cell. Nageli's 

 description affords no information as to the behaviour of 



* ' Neueren Al^ensyst.,' pp. 155—167, t. ii, f. 7—24. 



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