POLTGASTBIA. 



29 



are thrown off into the interior of the parent monadiary, which is 

 rent open to allow them to escape, as in fig. 15. 



Another mode of generation is by gemmation or the development 

 of bads, which in some species, as Cheroma, grow out of the fore 

 part of the body, and in others, as Vorticella, from the hind part, 

 near the stem, or from the stem itself, from which the young animal 

 soon detaches itself. In most VorticeUidce, as in Carchesium and 

 Epistylis, the small liberated end of the body opposite the mouth is 

 provided with a circle of vibratile cUia, so long as the individual swims 

 freely : but these disappear when the pedicle is developed. 



In the Zygnema, a freshwater confervoid Alga, the filaments of 

 which it is composed are developed, separately, by a linear multipli- 

 cation of cells ; the filaments then approximate, protuberances are 

 formed from corresponding cells of each, and these meet and adhere : 

 the co-adapted parts of the cell-walls disappear after a time, and the 

 contents of the confluent cells freely intermingle. This mode of de- 

 velopment is called " conjugation," and appears to be common to the 

 Algae generally. According to Stein*, the Gregarina conjugate, and 

 so form motionless spherical sacs, in which are developed a vast 

 number of minute bodies resembling XacicultE in shape, whence they 

 are called " navicella-sacs." But of these minute parasitic monads 

 more will be said in the Lecture on "Eutozoa." In the ciliated 

 Polygastria conjugation has been observed to take place in the genus 

 Actinophrys\f i. e., two individuals of A. 

 Sol, have been observed to unite, coalesce, and 

 become one. The same has been recorded 

 of species of Epistylis and of Vorticella. 



With regard to the more common fissi- 

 parous mode of multiplication, Ehrenberg 

 has figured gradations of this spontaneous 

 division of the organised contents of the 

 integument in the Gonium (Jig. 16.) and 

 Goninm. ChamydomoTias {Jig. 17.), which may be 



compared with the earliest stages of the development of the germ, 



as figured by Siebold in the 

 Strongylus and Medusa, by Baer 

 in the frog, and by Barry in the 

 rabbit; who, in 1841, remarked: 

 " On examining the figures given 

 by Ehrenberg of successive genera- 

 tions of the Chlamydomonas (Jig. 



Chlamydomoiiai. 



XXA^ p. 182. 



t XIX. p. 207. 



