54 



EMBRYOLOGY 



the ectoplasmic portion is separated from the endo- 

 plasmic (Fig. 20 B). The latter constitutes the ento- 

 dermic elements. The result of this process, which takes 

 place over the whole periphery, is the formation of a closed 

 two-layer cellular sac (Fig. 20 C), the outer layer of 

 which represents the ectoderm, the inner the entoderm, 

 while the central space, the former blastocoele, now becomes 

 the gastrocoele or archenteron. Soon there is a secretion of 

 a ti-anspareut jelly between the two layers (Fig. 20 C, g). 

 Since the embryo from this time forward swims about by 

 means of the flagellate motion of the ectodermal cells, this 



Fig. 20. — Three stages in the development of GeryonidEe : A and C from Liriope 

 mucronata (after Metschnikopf) ; B from Geryonia fungiformis (after Pol). A, six- 

 teen-cell stage ; h, cleavage cavity ; B, beginning of delamination ; C, after com- 

 pletion of delamination ; g, gelatinous substance. 



stage may be compared with the planula-stage of the other 

 Hydroids. 



The next change consists in the increase of the gelatinous 

 secretion, whereby the ectoderm sac is greatly distended. 

 Inasmuch as this secretion does not take place unifoi'mly on 

 all sides, the entoderm sac becomes more and more eccentric 

 until it touches the ectoderm at one point, the oral pole 

 (Fig. 21 A). The cells of the ectoderm and entoderm at 

 this place become thickened, and here the mouth-opening is 

 subsequently formed by the breaking through of these layers. 

 On the thickened ectodermic plate surrounding the mouth 

 there is soon established a special thickening of the peripheral 

 parts, whereby a circular wall is produced, on the outer side 

 of which the four (or six) primary tentacles are developed 



