THE BLASTULA. 59 



The interesting blastula germ-form is also of great sig- 

 nificance, for the modification of the mulberry -germ into the 

 germ-membrane vesicle takes place in the same way in a 

 great many animals of very dissimilar tribes ; for instance, in 

 many Plant-animals and Worms, in the Ascidians, in many 

 Star-animals (Echinoderma) and Soft-bodied Animals 

 (Mollusca), and also in the Amphioxus. In those animals, 

 however, in the ontogeny of which there is no real palin- 

 geiietic blastula, this deficiency is evidently only the result 

 of kenogenetic causes, of the formation of a nutritive yelk ;. 

 and of other conditions of embryonic adaptation. We may 

 therefore assume that the ontogenetic blastula is the repro- 

 duction of a primaeval phylogenetic ancestral form, and that 

 all animals (with the exception of the lower Primitive 

 Animals) have originated from a common parent-form, the 

 structure of which was essentially that of a germ-mem- 

 brane vesicle. In many lower animals, the evolution of the 

 blastula takes place not within the egg-coverings, but out- 

 side this, free in water. Very soon after this, each cell of 

 the germ-membrane begins to extend one or more movable, 

 hair-like protoplasmic processes; owing to the fact that 

 these cilia or whips vibrate in the water the whole body 

 swims about (Fig. 171, F}. This vesicular larva, the body- 

 wall of which forms a cell-stratum, and which rotates and 

 swims by means of the united vibrations of the cilia, has, 

 ever since the year 1847, been called the planula, or ciliated 

 larva. This designation, is, however, used by different 

 zoologists in different senses, and the gastrula, of which we 

 shall speak presently, has, especially, often been confused 

 with the planula. It is, therefore, more convenient to call, 

 the true planula-form the blastula. 



