H36 T- ^- Cary, 



place tlirougli the activity of certain of tlie segmentation cells. After 

 the segmentation has gone on for for some time, so that gastrulation 

 is iiearl}' completed, a few of the nuclei in the ectoderm become 

 distinguishable from the others through their smaller chromatin con- 

 tent and clearer appearence. At first the}' form a distinct layer 

 about the ectoderm (the segmenting eg^y is in this stage surrounded 

 by a mass of j^olk cells), later the "Hüllmembran cells" migrate 

 through the yolk and form a layer the over surface of the latter, 

 Avhich is thus brought inside the body of the embryo. 



From the above account it is apparent that in the only stage 

 (the development of the embryo from the fertilized egg) in which 

 the development was studied, the formation of the investing mem- 

 brane took place in the usual manner. So it seems all the more 

 improbable that in the same species two different methods of the 

 formation of the same organ would be found. 



In the Heterocotylea, according to Halkin (1902), Goldschmidt 

 (1902, 1905) and Kathaeiner (1904), the enveloping membrane arises 

 in an entirely different mannei'. 



In Zoogonus mirus, according to Goldschmidt, the egg capsule 

 contains an egg cell and two "primitive yolk cells", which have 

 come from the vitellarium. These last mentioned cells are not yolk 

 cells which function only by giving nutriment to the embryo; but, 

 on the contrary, from them is formed the investing membrane about 

 the developing embryo. These cells have the power of dividing 

 once, so that the membrane is composed of four cells, just as in the 

 embi'yos from the parthenogenetic eggs in the Malacocotylea. 



In the general part of his paper Goldschmidt (1905) discusses 

 the relationship of the investing membrane and the ectoderm in 

 trematodes. He reaches the conclusion that in embrj^os from ferti- 

 lized eggs the "Hüllmembran" is never formed from segmentation 

 cells. It is formed either by the rearrangement of the yolk nuclei 

 (nuclei of the yolk cellsj, as in Monostonmm, or by the spreading 

 over the embryo of the "rudimentary yolk cells", as in Zoogonus. 

 The so called "primary ectoderm" of the older writers is thus not 

 a part of the embryo at all. 



The casting off of the secondary ectoderm, as described by 

 Schauinsland for Distomum cylindraceum and D. mentulatuni, Gold- 

 scHMiDT characterizes as inconcevable, since in most miracidia, the 

 body cavity lies directly under the ectoderm. 



In the development of the embryos (germ balls) from partheno- 



