TREMATODES I DEVELOPMENT 139 



The spermatozoa do not differ essentially in their structure 

 from those of other animals ; the germinal cells or ova are cells 

 without integument and which contain a large nucleus and a 

 little protoplasm ; the vitelline sacs also yield nucleated cells, in 

 the plasm of which there are numerous yellow yolk granules ; the 

 yolk cells, like the germinal cells, detach themselves from the 

 ovarium and move into the excretory ducts to surround each 

 germinal cell in the ootype. They disintegrate sooner or later, 

 in the complete ovum, and there are utilised as food by the 

 developing embryo. 



DEVELOPMENT OF THE TREMATODES, 



(1) Copulation. Observation has demonstrated that the one or 

 two vaginae occurring in the ectoparasitic Trematodes are utilised 

 as female organs of copulation, and that the -copulation is alter- 

 native ; it is also known that Laurer's canal, which was formerly 

 generally indicated as the vagina, has never served the digenetic 

 Trematodes as such it appears to be homologous with the 

 canalis vitello intestinalis of the Monogenea but is the terminal 

 portion of the uterus termed metraterm. Reciprocal copulation 

 is observed as well as auto-copulation and auto-fecundation. The 

 spermatozoa accordingly wander through the entire uterus till 

 they reach the place of their final abode. 



(2) Formation of the Ova. The germinal cells are probably 

 already fertilised in the germinal ducts and then pass singly into 

 the space where they develop and where a large number of yolk 

 cells collect around them (fig. 74). The shell is then formed from 

 the secretion of the shell glands, which exudes in drops, the wall 

 of the oot} T pe meanwhile contracting considerably, and the ovum 

 moving into the uterus. The completed ova are of various forms 



correspondingly alter their direction. (2) The germinal trunk usually lies in front 

 of the testes, not rarely, however, behind them or between them. (3) The three 

 genital glands mostly lie together close in front of, or behind, the centre of the body ; 

 they may be moved far back, and may incidentally become separated one from the 

 other. (4) The vitelline sac may be single, in which case it also then lies in the central 

 field. (5) A few forms possess but one, others several or numerous testes. Amongst 

 the ecto-parasitic Trematodes there are also varieties with but one testis ; but they 

 mostly have several. As a rule, their uterus is short" but the ootype well developed. 

 The particular single or double canals (vagina) are used for copulation, not the uterus. 

 The vitelline ducts also communicate with the intestine through the canalis vitello- 

 intestinalis. 



