INVERSION. 191 



Fig. 22. — Germination of a Coral {M onoxenia Danvinit) : A, Monernla; 

 By Parent-cell (Cytula) ; C, two cleavage-cells ; D, four cleavage-cells ; 

 E, Mulberry-germ {Morula) ; F, the Germinal vesicle (Blastula) ; G, Ger- 

 minal vesicle in section ; H, Germinal vesicle (inverted) in section ; J, 

 Gastrula in longitudinal section; K, Gastrula, or Germ-cup, seen from 

 outside. 



In this Coral, as in many other low animals, the young 

 animal-germ begins to move even in this stage, and 

 swims about independently in the water. A long, thin, 

 thread-like process, a whip or thong, grows out from each 

 of the cells of the germ-membrane ; and these inde- 

 pendently exert slow vibrations, which afterwards be- 

 come quicker (Fig. 22, F). Each cell of the germ-membrane 

 is thus transformed into a vibrating whip-cell. The whole 

 globular germ-vesicle revolves or turns, and is driven about 

 in the water by the united force of all these vibrating whip- 

 processes. In many other animals, especially in those in 

 which the germ is developed within closed egg-membranes, 

 the vibrating whip -threads on the cells of the germ-mem- 

 brane are not developed till a later period, or, even, are not 

 formed at all. The germ- vesicle is capable of growing and 

 extending, for the cells of the germ-membrane increase by 

 repeated division, which occurs within the surface of the 

 ball, and more liquid is secreted in the centre cavity. 



A most important and remarkable process now occurs ; 

 this is the inversion of the germ-vesicle (invaginatio blas- 

 tulce, Fig. 22, E). The ball, the wall of which is cellular, 

 consisting of a single layer, changes into a cup with a two- 

 layered cellular waU. (Cf. Fig. 22, G, H, I) The outer sur- 

 face of the ball becomes flattened at a particular point ; and 

 this flattening deepens into a groove. The groove becomes 

 deeper and deeper, growing at the expense of the central 



