STRUCTURE OF THE OVARIES. 707 



end, -where they sometimes measure only 0-006 mm. (nucleus = 

 0-0045 mm.). 



The ova pass from the oviduct into the commencement of the 

 uterus through the fertilising canal, in -which they acquire their final 

 form by being equipped with yolk and shell. The fertilisation probably 

 takes place previously, during the passage through the fertilising 

 canal, as I conclude from the fact that I was but rarely able to 

 discover spermatozoa within the uterns. 



The yolk-spheres, which surround the ovarian ova in large 

 numbers (from 25 to 30), are seen to be also membraneless nucleated 

 cells, having, however, a decidedly smaller size (at most O'Ol mm.), a 

 smaller nucleus (0-004 mm.), and a plasma containing numerous 

 fine and coarse granules. Eschricht correctly recognised the seat of 

 formation of these spheres in the so-called " granular heaps," which, 

 with their efferent apparatus from the yolk-gland of Bothriocephalus, 

 may be most fitly termed " yolk- vesicles." 



As already noted, these structures belong to the cortical layer, in 

 which they extend in a simple close-packed layer beneath the sub- 

 cuticula (Fig. .361). They are found both on the dorsal and ventral 

 surface, but only in the lateral areas, the median region remaining for 

 the most part free. The yolk-vesicles in the middle area, especially 

 on the ventral surface, are, however, considerably larger than 

 those more peripherally situated, some of which do not measure 

 more than 0-04 mm., while the former are sometimes six or eight 

 times as large, so that they become recognisable by the unaided eye, 

 especially since their contents render them non-transparent. The other 

 yolk-vesicles are, of course, also non-transparent, but in consequence 

 of their small size, this is only expressed by the darker shade of the 

 lateral areas in which they are situated. In their form they some- 

 what recall the testicular vesicles, with which Kiichenmeister long 

 regarded them as identical. The contents and situation are, however, 

 quite different, as also the closer disposition, so close in some places 

 that the spheres coalesce. In longitudinal sections I coimt, like 

 Stieda, about thirty of these in transverse section, about forty-five to 

 fifty on each side on the dorsal and ventral aspect, so that the total 

 number in each joint must be near 6000, as Eschricht also estimated. 

 No special membranous boundary is recognisable. They are simply 

 collections of cells and granules, but the former have not from the 

 first, especially in young joints, their subsequent size. 



Since the yolk-vesicles are destitute of distinctly independent 



boundary, their contents can only be expelled by the contraction of 



the body-muscles. Especially the sagittal fibres are concerned in 



this action, for they extend between the vesicles which owe to them 

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