27(i 



ZOOLOGY 



embryonal pharynx. Meanwhile a group of four cells enclosing a 

 cavity are modified to form the foundation of the endoderm, 

 and increasing in number give rise to the embryonal intestine, 

 into which the embryonal jiharynx soon opens, the latter opening 

 on the exterior by a mouth aperture. 



The embryonal pharynx (Fig. 218, pli,') has the function of 

 swallowing the yolk-matter with which the embryonal intestine 

 becomes greatly distended. At a subsequent stage the embryonal 

 pharynx and intestine are aborted, and the former comes to be 

 represented merely by a mass of cells. In this a cavity arises 

 — the cavity of the permanent pharynx. The permanent intestine 



Fio. 218.— Sections through embryos of Dendrocoelum lacteum (somewhat diagrammatic). 

 dz, yolk-cells ; ec, ectoderm ; en, endoderm ; -pk', embryonal phai-ynx ; ph", permanent 

 pharynx : loz, wandering cells. (From Korschelt and Holder, after Hallez.) 



becomes formed and the cavity of the pharynx opens into its lumen. 

 Subsequently the permanent mouth makes its appearance. The 

 brain is formed in the thickness of the blastoderm, and thus 

 appears to be of mesodermal origin, not of ectodermal, as in 

 the Pclyclads. 



In some- Rhabdocceles (certain species of Mesostoma) two distinct 

 kinds of eggs are formed — summer and winter eggs. The oosperms 

 are deposited singly, and each, together with a mass of yolk-cells, is 

 enclosed in a chitinous, usually stalked, shell. Segmentation 

 takes place very much as in the Polyclads. No embryonal 

 pharynx is formed, the permanent pharynx performing the function 

 of swallowing the yolk-matei'ial : it appears to be of endodermal 

 derivation. The intestine arises from a group of cells — the 



