VOLVOCINE^E. 59 



greater part of the time it is regularly in the direction indicated, and 

 the point of rupture of the sphere ivill be at its north pole. 



" It is difficult to determine precisely how this rapture is accomplished, 

 but I believe it to be by a special contraction of the walls of the parent, 

 or of the invisible primordial utricle, not by the outward pressure of 

 the daughter spheres, this force being evidently inadequate to produce 

 the result where their number is small, whatever it may be when it 

 reaches its maximum. 



" Shortly before the emission of the young the cell commonly assumes a 

 slightly pyriform. shape, and then slowly opens at its apex, but the aperture 

 is of less diameter than that of the young Volvoces, and as each of these 

 passes out, the month of the bag is visibly stretched, and resumes its 

 original size after each daughter sphere has escaped, so that it evidently 

 possesses considerable elasticity, a property also made manifest by the 

 fact that the normal form of Volvox may be considerably flattened by 

 the pressure of a glass cover, and yet resuoie both its sphaerical form and 

 its motion when this pressure is removed. 



" Moreover, the daughter sphere passes out ivithout rotating, and from 

 whatever cause it derives its impulse, this often suffices to drive the 

 young Volvox clear of the mouth of the sac to a distance equal to 

 several times its own diameter, in which position it pauses motionless 

 for some seconds, and then, commencing to rotate gently, sails away, at 

 first slowly, then more and more rapidly, to enjoy its independent 

 existence. 



"After the rupture of the sac, the gonidia near the edges of the 

 opening are seen to quiver, from the action of the cilia, where they are 

 partially freed from the support of the surrounding envelope, and the 

 same thing occurs when they are forcibly torn from their attachment, 

 in which case they may even move for awhile freely through the 

 water. 



"The general action of the cilia continues for some time, and the 

 empty sphere rotates as before, its general direction being still from 

 north to south, with the open end to the rear. After a time, which I 

 cannot specify, the cilia cease to play, and the organism decays, having 

 fulfilled its destiny in life. 



" The birth of the young Volvocesis affected by various circumstances. 

 Doubtless the process is, under natural conditions, most active in the 

 early hours about dawn, when the analogous functions of similar 

 organisms are well known to be most energetic, but in order to see the 

 phenomenon in full vigour it is only necessary to place a number of 

 mature parent- spheres, such as are found in every colony, in a shallow 

 live-trough, and to bring them into a warm room. In an hour's time 

 almost all the young plants will have been liberated. Light and heat 

 stimulate the action, while cold and darkness retard it. The ciliary 

 action is aff acted in a remarkable degree by altered external conditions. 

 It' a drop of water considerably colder than that in which the Volvoces 

 are floating be allowed to flow in under the cover-glass, the whole are 

 paralysed for some seconds, after which they slowly resume their motion. 

 A sudden mechanical shock produces a similar effect. A sufficient 

 degree of heat to make the water distinctly tepid to the feel causes 

 instant and simultaneous death of the whole colony. 



" During the day the majority of the Volvoces contained in a shallow 

 vessel rise to the surface, although they avoid strong direct sunshine, 

 while at night they retire in a cloud to the bottom. 



" The astonishing number in which the spheres at times appear in 

 some pool, and their equally sudden disappearance, have been frequently 

 remarked. Doubtless a very slight change in external conditions suffices 

 on the one hand to favour the development of countless thousands of 



