The life history of Diplodisciis temporatus Stafford. 639 



Zeller (1876) describes for Polystomum integerrimum, a course 

 of development which is clearly a case of epibolic gastrulation. 



Schauinsland (1883). on whose description of the development 

 in several species of trematodes most text book descriptions are 

 based, i-ecognized the formation of a morula, but could distinguish 

 no trace of gastrulation or formation of germ layers. 



Within recent j-^ears, a true gastrulation has been described in 

 the development of several trematodes. 



Halkin (1901) and Goldschmidt (1902) describe the gastrulation 

 of Polystomum. Kathariner (1904) finds essentally the same process 

 taking place in Gyrodadylus elegans. 



Goldschmidt (1905) describes a very unusual method of for- 

 mation of the germ layers in Zoogonus mirus. In this specis there 

 is cut off from one cell of the twelve celled morula, by "perikline 

 Teilung" a cell: "die nunmehr das Zentrum des Embryos einnimmt 

 und als primäre Entodermzelle anzusprechen ist. Indem diese pri- 

 märe Entodermzelle sich in zwei hintereinanderliegende Zellen teilt, 

 ist die Bildung der primären Keimblätter vollzogen. Ein aus 

 2 Zellen bestehendes Entoderm wird von einer Schicht kubischer 

 Ectodermyellen umgeben, die sich ihrer Größe und ihrem Bau nach 

 als verschieden wertig erweisen." 



Ortmann (1908) describes the usual gastrulation as taking place 

 in the development of Fasciola Jiepatica. In his figures of the later 

 stages of segmentation, before the etablishment of the primordia of 

 the organs of the miracidium, all cell outlines become obliterated so 

 that the arrangement of the layers cannot be made out. 



In Diplodisciis temporatus the endoderm cells retain their identity 

 for some time after all cell walls have dissappeared in the ectoderm. 

 Even after the outlines of the endoderm cells have been lost, the 

 line of demarcation between the entoderm and ectoderm remains 

 clearly distinguishable for some time (fig. 53). In some of the 

 embryos the cells making up the endoderm remain distinctly marked 

 oif from the other cells throughout the segmentation, and until the 

 characteristic arrangement of the endoderm to form the primordium 

 of the gut has taken place. In the greater number of the embiyos, 

 however, the distinction between the layers becomes lost in a com- 

 paratively early stage in the segmentation, and is not recognisable 

 again until the time when the primordium of the gut is diffe- 

 rentiated. 



Rossbach's contention that the cells which go to form the 



42* 



