STRUCTURE AND LIFE-HISTORY OP VOLVOX GLOBATOR. 213 



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 mouth 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 con- 

 siderably flattened by the pressure of a glass cover, and yet resume both 

 its spherical form and its motion when this pressure is removed. 



Moreover, the daughter-sphere passes out without 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 Volvoces is 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 affected in a remarkable degree by altered external conditions. 

 If 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, as I have repeatedly 

 seen to happen in consequence of the rude impact of some ferocious 

 Entomostracon. 



A sufficient degree of heat to make the water distinctly tepid to the 

 feel, causes instant and simultaneous death of the whole colony. 



