152 



LECTURE VIII. 



grapes in clusters by the pellucid (chorionic ?) membrane in which 

 they are enclosed {^fig. 73., 1.) ; the yolk is a "germ-yolk," and has a 

 73 vitelline membrane separated 



by a whitish fluid from the 

 chorion. The fission of the 

 germ-cell is followed by 

 total cleavage of the yolk, as 

 in 2 ; it next subdivides into 

 four, as in 3 ; after which the 

 formation of the germ-mass, 

 4, proceeds rapidly. Its sur- 

 face becomes smooth, large 

 cilia are developed from one 

 end, which becomes marked 

 off by a constriction from the 

 Development of Pedicciiina. other, as at 5. The larva 



escapes from the chorion under the form 6, and swims freely abroad. 

 The ciliated margin expands, and renders that end of the larva 

 funnel-shaped. Tubercles bud forth from the funnel, and a pedicle 

 is developed from the opposite end, as at 7 ; by this the larva attaches 

 itself, and in the course of the subsequent metamorphosis the linea- 

 ments of the parent Bryozoon soon begins to be traceable, as at 8. 



The development and vital phenomena of the reproductive gem- 

 mules have been studied, by Dr. Farx*e with much care and success in 

 the sponge-like Halodactylus {^Alcyonium gelatinosum, of Pallas). 

 They appear in spring as minute whitish points just below the surface. 

 If one of these points be carefully turned out with a needle, it is 

 74 found to consist of a transparent sac, containing 



generally from four to six of the larvas. These 

 are of a semi-oval form, with the margin of their 

 plain surface developed into tubercles support- 

 ing groups of vibratile cilia {Jig. 74.).* The 

 body presents a simple granular structure ; the 

 gemmule swims about actively by the vibration 

 of its cilia^ the motion of which seems to be 

 control. They generally swim with 

 convex part forwards ; sometimes they 

 upon their axis, or execute a 

 series of summersets ; or, selecting a fixed point, they whirl round 



under its 



Larva of Halodactylus, from ,. 



above; the cilia as when slowly tUC 



acting round the margin in . , . , 



waves. simply rotate 



* The luovemcnts of tho cilia, which give the appeai'ance of a succession of 

 waves, have been closely analysed in XXXV. pp. 410, 411. 



