Dr. F. Colin on a new genus of the fumihj 0/ Volvocinese. 323 



cumference of a circle (PI. VI. fig. 3 b, b), and of a large common 

 envelope, enclosing the former as a colourless vesicle, at the equator 

 of which are ranged the said eight green globes (fig. 3 a). 



The common envelope is bounded by a membrane wholly de- 

 void of structure and transparent, so that it may be overlooked 

 if the illumination be not properly modified, under which cir- 

 cumstances the eight green globes appear destitute of any 

 common bond of union. But the membrane of the envelope 

 always exists, and although very delicate and thin while young, 

 it becomes thickened with age, and then possesses an evident 

 breadth, albeit no compound structure can be detected. The 

 membrane of the envelope is absolutely rigid and never changes its 

 shape, excepting through the ordinary expansion of growth; 

 therefore it is not only totally devoid of contractility, but is even 

 elastic only in a slight degree. 



In whatever direction the total organism may lie during its 

 movements, the envelope always appears as a perfect, absolutely 

 regular circle (figs. 1, 2) ; thence it results most decidedly that 

 the membrane of the envelope forms a sphere, which may perhaps 

 deviate but very little from the mathematical ideal. The diameter 

 of the envelope varies between tolerably wide limits : while 

 younger forms possess an envelope some ^^q^^ °^ ^ ^^^® 

 (0-028 mm.) in diameter, most attain one of y'^th (0*044 mm.), 

 and the largest are as much as ^j^i^ of a line (0*055) in 

 diameter. 



The phsenomena in dissolution and during propagation prove 

 that the membrane of the envelope immediately surrounds a 

 colourless watery fluid, the refractive power of which does not 

 differ from that of water. The envelope may therefore be re- 

 garded as a broad, spherical cell with a delicate structureless 

 membrane, colourless and transparent like glass, containing a 

 ■thin, water-like, colourless fluid ; consequently I shall denominate 

 it the envelope-cell {Hull-zelle) . 



While the envelope-cell varies generally speaking only in 

 size, and no difference whatever of shape and structure can 

 be detected in the different individuals, the variations in the 

 development of the eight green globes in its interior are very great 

 (fig. S b,b). In fact it is difficult to represent the multiplicity 

 of forms which here display themselves, so as to give a full and 

 clear idea of them ; and our figures even can afford but a very 

 insufficient picture, since scarcely a single individual exactly re- 

 sembles another, in this respect. The eight green bodies in 

 the interior of each envelope-cell, which, for reasons to be given 

 hereafter, I shall caW primordial-cells, are in their simplest con- 

 dition globular, and stand at equal distances in a circle at the 

 largest circumference of the envelope-cell, so that the whole 



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