182 GENERAL HISTOR,Y OF THE INEUSOEIA. 



by Cohu, a close homology is perceptible. In this plant the membrane sui'- 

 roimding the hexagonal primordial cells gives off from each angle a tubular 

 process, which comes into intimate apposition at its extremity with that from 

 an adjoining cell. However, between these processes of Gonium and those of 

 Volvox, there is this diiference, that in the former it is only the primordial 

 membrane which is drawn out to form a canal or tubule, whilst in the latter 

 the protoplasm itself is at first extended with its membrane, and subsequently 

 collapses into a.delicate band. 



The circular contractile vesicle noticed by Ehrenberg has had its existence 

 confirmed by Mr. Busk in mature cells {T. M. S. 1852, p. 35) : — " It may be 

 situated in any part of the zoospore (XX. 35), not unfrequently in the base, 

 or even in the midst of one or other of the bands of protoplasm connecting it 

 with its neighbours ; it is pretty uniform in size, and about l-9000th of an 

 inch in diameter." Its most curious property is its rhythmical contractility, 

 its pulsations occurring very regularly at intervals of about 38" to 41". The 

 contraction is rapid, whilst the dilatation is gradual." The vesicle " would 

 seem to exist, or at all events to present a contractile property only for a 

 limited period, and to disappear soon after the formation of the brown spot," 

 ^. e. the eye-speck. The coloiu"ed eye-speck or stigma lies close against 

 the primordial-ceU wall, it is not invariably present, and consequently cannot 

 be esteemed of essential importance as a characteristic. 



The wall of a Volvo.vhas an appreciable thickness, represented by a vertical 

 section — in fact by the de23th of the cells, which are placed side by side, the 

 lines of junction being straight and perpendicidar to the external siuface of 

 the globe (XX. 36). The inner sides of the cells, bounding the internal cavity 

 of the globe, are somewhat convex, the result of mutual lateral pressure, and 

 the absence of centrifugal pressure. Prof. Williamson has well displayed 

 this by sectional diagrams taken from his preparations. These sectional views 

 also demonstrate the position of the rounded masses of green protoplasm — 

 the primordial sacs — to be immediately on the inside of the peripheral mem- 

 brane or envelope-cell of the Volvo.v (XX. 37, 38). 



Develojmient of Volvox. — Self-division of the primordial cells, or zoospores 

 (Busk), of Volvox is regulated by the same laws that prevail in other Volvo - 

 cinece and in other imicellular Algae in general. Among the yoimger speci- 

 mens of Volvox, one or more larger globules are observable (XX. 42-44), which, 

 if watched, will be found to undergo segmentation, fii^st into two (XX. 42), 

 then into foui' portions (XX. 43), and so on (XX. 46), alwaj's keeping to the 

 power of two and its multiples, until some hundreds of minute corpuscles are 

 developed (XX. 47), which, according to the nature of the genus, so dispose 

 themselves in a lamina as to enclose a hollow spherical space, and to assume 

 the characteristic globular form. Thus a new Volvox is generated, but difi'er- 

 ing from mature forms in the contiguity of its component individuals, — a differ- 

 ence, however, which progressively vanishes with advancing age. The young 

 globe lies immediately within the parent being, to which for a time it adheres, 

 as it would seem, by means of a delicate capsular membrane, within which 

 its development has proceeded. This indeed forms its sole bond of union with 

 the common envelope of the parent Volvox (XX. 33). 



When first formed, the cilia of the primordial cells do not penetrate through 

 the external envelope of the young globe : however, this condition is of short 

 dui^ation ; for no sooner is the detachment from the wall of the parent about to 

 supervene, than the cilia protrude externally, and, commencing their vibratile 

 movements, soon set the newly-developed colony in motion within the cavity 

 of its parent. The detachment is consequent on the rupture of the investing 

 capsule, caused, no doubt, by the constantly enlarging bulk of the young 



