CNIDARIA 



41 



zone being less Toluminous than those of the vegetative. In 

 early stages the blastomeres move away from the centre, so 

 that there is formed a gradually enlarging cleavage cavity 

 within. By additional but less regular cleavages the blasto- 

 meres increase in numbers, and arrange themselves into a 

 single layer of epithelial cells surrounding the cleavage 

 cavity, thus attaining the typical hlastula- stage (Fig. 13 D). 

 This cell- vesicle now elongates, so that it becomes ovoid, or 

 sausage-shaped ; and its surface becomes covered with flagella 



Fig. 14.— Formation of the entodenn by polar ingressioa in the planulae of 

 Aequora (after Glaus, from Hatschbe's Lelirhuch). 



(Fig. 13 B), by the motions of which it swims about with 

 the broader end of the body directed forward. The forma- 

 tion of the entoderm now takes place by polar ingression, at 

 first a few, and then numerous, cells migrating from the 

 posterior end of the body into the cleavage cavity, so that, 

 advancing from behind forward, they gradually fill it up 

 (Fig. 13 E, Fig. 14). In this way there arises a larva which 

 is eminently characteristic for the Hydroids, and was named 

 by Dalyell the planula (Fig. 15 A)-, it has also been called 

 a parenchymula, on account of the embryonic cell-mass filling 

 its interior (Metschnikoff). During further development 

 there are formed in the ectoderm nettle-capsules, which seem 

 to be especially concentrated about the posterior pole, while 

 within the mass of entoderm cells there arises a fissure, the 

 first trace of the gasiral cavity^ around which the entodermal 

 cells assume an epithelial arrangement. Preparation is now 



