98 INVERTEBRATA CHAP. 



originates close to one side of the larva, not at the absolute edge as it 

 should do were the embryo a perfect half; in consequence of this 

 fact the growing stomodaeum indents the endodermic sac unevenly, 

 cutting off a little third pouch. 



From the fact also that in this half larva, on the side next where 

 the missing half should be, the endoderm is not naked but covered 

 with ectoderm ; and from the fact that by separating the lower 

 ends of the macromeres he succeeded in producing a form with two 

 stomodaea, Fischel concludes that although the material destined to 

 form the specialized ribs and the apical plate is originally rigidly 

 localized in the germ, this is not the case with the rest of the ectoderm. 

 This unspecialized ectoderm spreads till it covers all the exposed 

 macromeres, whilst the stomodaeum is formed owing to the action 

 on this general ectoderm of a stimulus proceeding from the lower 

 ends of the macromeres. 



From a review of the work of all these experimenters we draw 

 the conclusion that in the eggs of Hydro medusae, up to the 16- 

 cell stage, the part resembles the whole in its constitution ; and the 

 question whether one portion shall form only part of an embryo or 

 whether it shall form a whole embryo, depends on whether or not it 

 remains in connection with its fellows. In Ctenophora, however, we 

 must conclude that the part is quite different from the whole, and 

 that the parts destined to form the ribs and apical plate are mapped 

 out even in the uiisegmented germ ; these parts must, therefore, 

 be represented by portions of the cytoplasm of the egg, not by nuclei. 



The egg is incapable of regenerating a lost part, but in this 

 respect a remarkable observation of Chun's (1880) must be borne in 

 mind. He experimented with the eggs of Bolina and obtained half 

 larvae just as did Driesch and Fischel with Beroe. But Chun kept 

 his half larvae living for weeks, till they developed generative organs 

 on their meridional canals, and he states that the missing half was post- 

 generated. This observation has been recently confirmed by Mortensen. 



That an animal in its young stages should possess no powers of 

 regeneration, but should acquire them when it is older, is a curious 

 fact but not unprecedented, for the same thing occurs in Ascidians. 

 We have seen that the specialization of the egg for the formation of 

 organs is a specialization of plasma, but it will be shown later in this 

 volume that this specialization must lie regarded as due to an 

 influence emanating from the nucleus of the ovum during the period 

 of its growth and ripening. We must, I think, attribute the 

 secondarily acquired power of regeneration to a reorganization of the 

 cytoplasm, due to renewed influences emanating from the nucleus. 



ANCESTRAL MEANING OF THE PLANULA 



We have now to consider the meaning of the development of 

 the planula. We have already put forward the hypothesis that 

 the planula represents a free-swimming ancestor common to all 



