FASCIOLA HEPATICA, 165 



inch in their long diameter. Below the point in question the 

 extreme upper end of the oviduct is developed into a special pouch- 

 like organ, known as the posterior seminal vesicle of Yon Siebold. 

 In this receptaculum seminis the threads lie coiled together en masse, 

 and appear to perform the very act of impregnation as the egg- 

 germs pass downwards into the commencement of the oviduct. 

 Having once arrived at this situation, the first envelope of the 

 egg is developed from a secretion furnished by a special shell-gland, 

 and during the passage of the ovum along the oviduct, into which 

 it has now entered, a second finer shell is formed upon the first. 

 According to Leuckart, the horny chitinous secretion subsequently 

 deposited on the outer shell-membrane is derived from another 

 special set of chitine-secreting gland-cells, lining the oviducal 

 canal. The same authority adds, " Both the membranes refract 

 light highly, but they present a different hue. The inner one being 

 green and the outer one of a reddish tint, the usual yellow colour 



"Fia. 36. — Egg of JSasciola hepatica in an early stage of yelk-segmentation (X 280 diam.). — Original. 



of the Qgg undoubtedly owes its origin to the mixture of these two 

 colours." This also accounts for the marked differences in the 

 degree of colouring observed according to their situation in the con- 

 tinuous oviducal and uterine canal. Pale and almost colourless at 

 first, it is only when they have reached the central uterine passage 

 that they have finally acquired their full, bright, brownish-yellow 

 colouring, 



At the time of their highest uterine development or period of 



