CNIDARIA 



41 



zone being less voluminous than those of the vegetative. In 

 early stages the blastomeres move away from the centre, so 

 that there is foi'med a gradually enlarging cleavage cavity 

 within. By additional but less regular cleavages the blasto- 

 meres inci-ease in numbers, and arrange themselves into a 

 single layer of epithelial cells surrounding the cleavage 

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

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

 sausage-shaped ; and its surface becomes covered with flagella 



Fig. 11.— Formation of the entoderm by polar ingression in the planul® of 

 Aequora (after Claus, from Hatschek's Lehrhuch). 



(Fig. 13 -£/), 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 -£/, Fig. 14). In this way there arises a larva which 

 is eminently characteristic for the Hydi'oids, 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 gastral cavity, around which the entodermal 

 cells assume an epithelial arrangement. Preparation is now 



