PLATHELMINIHES 



171 



surrounding' homogeneous food-mass, and by becoming flat- 

 tened out and uniting with one another form there a thin 

 membrane [ectoderm]. Later a small group of cells in the 

 mass of loosely associated embryonic elements becomes dis- 

 tinguishable by its presenting a more compact arrangement. 

 This spherical group of cells at first lies in the middle of the 

 embryonic mass, but later moves to the periphery. Here it 

 unites with the ectoderm. It then becomes hollow, and its 

 cells are differentiated into various layers, thus forming the 

 provisional organ known as the embryonal pharynx (Fig. 

 85 A). Four cells, which enclose a small space, are applied 

 to the inner end of this pharynx. According to Hallez, these 

 constitute the earliest fundament of the intestine (Fig. 85 A). 

 The fundaments of the pharynx and intestine would be 

 looked upon, then, as entodermal, but the migratory cells 

 which remain between ectoderm and entoderm could not be 

 designated as mesoderm, for later, according to Hallez, 

 ectodermal as well as entodermal elements arise from them. 

 These migrator}^ cells contribute at first to the formation of 

 the musculature of the embryonal pharynx, becoming elon- 

 gated and spindle-like, and being applied to its outer side. 



The significance of the pharynx, which now begins to 

 execute swallowing movements, consists merely in its trans- 

 porting the yolk-cells into the embryo (Fig. 85 B). As soon 

 as the pharynx begins to function, the intestine becomes 

 rapidly filled with yolk-cells, which cause a great distension 

 of the intestine and the entix^e embryo. The inconsiderable 

 entoderm and likewise the ectoderm become stretched to 

 an extraordinary degree, so that they can be recognized only 

 with difficulty. In order to prevent a bursting of these thin 

 layers, cells derived from the migratory elements unite with 

 them. Metschnikoff's statement that yolk-cells which have 

 migrated in from the outside are converted into the epithe- 

 lium of the intestine is not corroborated by Hallez. Accord- 

 ing to this observer, the primitive entoderm always forms a 

 wall, although a very delicate one, bounding the parenchy- 

 matous tissue of the embryo. This entoderm, to be sure, is 

 said to be of a provisional natui-e only. It disappears sub- 

 sequently, and the adjoining migratoiy cells unite to form 



