14 ILLINOIS BIOLOGICAL MONOGRAPHS [14 



EMBRYOLOGY 



"Larval trematode" has been used for any phase of the life-history from the 

 fertilized egg to the adult trematode. This is truly a wrong conception in 

 the hght of the observations of investigators from early times down to the 

 present. Workers have recognized sporocyst and redia as "nurse" to the 

 progeny. In other words, they are parthenogenetic individuals. Ssinitzin 

 (1905, 1910, 1911) has made a wise separation of sporocyst and redia on the 

 one hand from the cercaria on the other. He groups the mature sporocyst 

 and redia together under the term parthenita, or parthenogenetic mother. 

 This term is used thruout this paper to define the mature sporocyst and redia 

 as distinguished from cercaria or other larval trematode of the hermaphroditic 

 generation. 



The homology between cercaria, sporocyst and redia in their early stages 

 of development is recognized by Schwarze (1886:64), who compares his studies 

 on the cercaria's development with those of Schauinsland (1883) on miracidia. 

 Schwarze notes the similarity of the "cuticula," the sloughing off of the epider- 

 mis, the location of the solid entoblast, which divides so that a portion comes 

 to he next to the ectoderm and another part around the gut. He shows that 

 the homology is very apparent. "Die Keimzellen des Embryos entsprechen 

 den Genitalzellen der Cercarie, die etwas abgeplatteten Epithelzellen des 

 Embryo den MeristemzeUen der Cercarie." He observes the similarity of 

 the excretory system in miracidia, redia and cercaria, and adds "ferner sind 

 sie ebenfalls mit einem Nervensystem ausgestattet, welches die grosste Aehn- 

 lichkeit mit demjenigen der Cercarien hat." 



Since the miracidium, redia, and cercaria are not, in last analysis, three 

 parts of one life-history, but more exactly three or more genetically related but 

 complete Hfe-histories, it would not be too much to expect, then, that the 

 origin of germ layers in miracidium, redia, and cercaria would be the same. 

 All three generations arise by the cleavage and development of a single germ 

 cell of mesodermal origin. In the miracidium the cell is always fertilized; 

 in the redia and cercaria it is always parthenogenetic. The observations on 

 the similarity of origin of these cells are extensive, yet mostly isolated, and 

 deserve re-emphasis. 



The exact method of development of individuals within the sporocyst or 

 redia has been a matter of diversity of opinion. In general one of two views, 

 budding or parthenogenesis, has been supported by investigators. The 

 earHer writers who considered the origin of the germ balls described them as 

 arising endogenously. Thus Moulinie (1856:132) writes: "les Cercaires 

 naissent, comme nous I'avons vu en parlant des Sporocystes, de gemmes plus 

 ou moins arrondis qui se forment dans I'interieur de ses derniers lorsqu' ils 

 atteint leur developpement normal. " Then the question arose whether or not 

 the germ ball arose from the ordinary tissues lining the body cavity of sporocyst 

 or redia, or v^^hether special cells were set apart as a germinal epithelium. 

 Leuckart (1886:113-125) asserts that in all cases germ balls arise only from 



