41 t THE REPRODUCTIVE FUNCTIONS. 



cease to grow. The Lemna^ or common Duckweed, which, 

 consists of a small circular leaf, floating on the surface of 

 sta«-nant pools, presents a singular instance of the develop- 

 ment of germs from the edges of the leaves, and the suhse- 

 quent separation of the new plant thus formed. In this re- 

 spect the process is analogous to the natural mode of multi- 

 plication met with in the lower orders of Zoophytes, such as 

 the Hydra. At the earliest period at which the young of 

 this animal is visihle, it appears like a small tuhercle, or bud, 

 rising from the surface of the parent hydra: it grows in this 

 situation, and remains attached for a considerable period; at 

 first deriving its nourishment, as well as its mechanical sup- 

 ])ort, from the parent; then occasionally stretching forth its 

 tentacula, and learning the art of catching and of swal- 

 lowing its natural prey. The tube, which constitutes its sto- 

 mach, at first communicates by a distinct opening with that 

 of its parent: but this opening afterwards closes; and the fila- 

 ments by which it is connected with the parent becoming 

 more and more slender, at length break, and the detached 

 hydra immediately moves away, and commences its career 

 of independent existence. This mode of multiplication, in 

 its first period, corresponds exactly with the production of 

 a vegetable by buds; and may therefore be classed among 

 the instances of gemmiparous reproduction; although at a la- 

 ter stage, it differs from it in the complete detachment of 

 the offspring from the parent. 



Another plan of reproduction is that in which the germs 

 are developed in the interior of the animal, assuming, at the 

 earliest period when they become animated, the form of the 

 parent. In this case they are termed gemmules instead of 

 buds. This mode of reproduction is exemplified in the Vol- 

 vox, which, as we have already seen, is an infusorial animal- 

 cule of a spherical form, exhibiting incessant revolving move- 

 ments.* The germs of this animal are developed, in great 

 numbers, in its interior, having a globular shape, and visible^ 



• Vol. i. p. 159. This animal is delineated in Fig. 79. 



