The life history of Diplodiscus temporatus Stafford. 637 



genetic eggs of the Malacocotylea the investing membrane is, in all 

 known instances, formed from two of the micromeres ot an early 

 stage in the segmentation. In Diplodiscus, and in other forms where 

 the early segmentation has been described, it appears that the 

 second cleavage separates two cells which are to give rise to the 

 ectoderm from one cell which is to give rise to the endoderm and 

 mesoderm. The cells which go to form the investing membrane are 

 thns clearly ectodermal cells which in an early stage in the deve- 

 lopment of the embryo become set apart for the performance of a 

 special function, as a protective and osmotic membrane. 



That these four investing cells represent the entire ectoderm, 

 as has been maintained by Eossbach, cannot, in the light of our 

 knowledge of the segmentation of both the fertilized and partheno- 

 genetic eggs of trematodes, be admitted. Another assumption of 

 this theory, i. e. that all of the cells between the primordium of 

 the gut and the investing membrane constitute the mesoderm, violates 

 a firmly e.stablished principle of comparative embryology, in that it 

 necessitates the origin of the central nervous system and the epi- 

 dermal sensory organs from the mesoderm. 



In the embryonic development of many other animals, parti- 

 cularly the worms and molluscs, certain cells among the micromeres 

 become early set apart for the foi'mation of larval organs, which in 

 the further development of the embryo cease to be functional and 

 are either resorbed or cast off. In the investing membrane of the 

 embryo from the parthenogenetic eggs of Malacocotylea we have, in 

 my opinion, an exactly parallel case, and one, therefore, which in no 

 way concerns the question of the formation of the germ layers. 



According to the observations of Tennent (1905) and Rossbach 

 (1906) with which my own agree, the investing membrane persists 

 about the embryo until the formation of the permanent cuticle of the 

 cercaria takes place. On the formation of the last mentioned struc- 

 ture the investing membrane is no longer useful and it becomes 

 buried in the secretion from the cuticular glands, probably becoming 

 a part of the permanent cuticle, or perhaps forming the basement 

 membrane. 



In considering the development of the embryos from partheno- 

 genetic eggs (germ balls) few investigators have given any special 

 attention to the formation of the germ layers. Thomas (1883) des- 

 cribes a true invagination as taking place in the germ balls of 

 Fasciola hepatica, which are to form rediae. 



Zool. Jahrb. XXVIII. Abt. f. Anat. 42 



