THE SHELL-GLAND AND YOLK-GLAND. 315 



the height of the sperm sac, and also somewhat below it, on the right 

 and left of the middle line, and may be beautifully and distinctly brought 

 out by injected microscopic preparations. Constructed on the type of 

 the tubular glands, they consist, especially in the larger species, of nume- 

 rous more or less branched tubes, which contain the egg- cells enveloped 

 within an extremely fine structureless membrane, in the form of clear, 

 membraneless, little balls, with comparatively large germinal vesicles. 



But it is quite different with the yolk-gland (albuminous gland, 

 Sommer), not only because, although usually an unpaired organ, it is 

 sometimes drawn out to the side, but because its efferent canal opens 

 directly into the shell-gland, into which there are also poured semen 

 (Figs. 163, 164), eggs from the ovary, yolk, and shell material, and 

 which possesses all the conditions necessary for the later formation 

 of the eggs. The yolk -gland is situated near the posterior extremity of 

 the joint, and thus below the other parts of the female apparatus. In 

 the larger species it consists of a branched glandular body like the ovary ; 

 but in other cases it is of a simpler saccular shape. The very fine and 

 structureless walls enclose little cells, which in the young joints often 

 contain several nuclei, but which afterwards generally dissolve, and as- 

 sume the appearance of a somewhat thick and tough glandular secretion. 



The manner in which the position and arrangement of the above 

 shortly described organs is affected by the form of the joint is well 

 shown in the structure of Tcenia perfoliata, which we may regard as 

 typical representative of the short and broad-jointed Taeniadse. It is 

 most noticeable in the ovaries, which, in contrast to their usual struc- 

 ture, appear as two thin canals, which run out from the middle line 

 towards the edges, and are provided throughout with short unbranched 



FIG. 166. Male and female organs of Tcenia perfoliata (after Kahane). ( x 15.) 



egg-follicles (Fig. 166). They have thus a remarkable resemblance to 

 the male secretory apparatus, which is also determined by the form of 

 the body, and runs in the same direction, and to pretty nearly the 



in the former edition of this work is also the first complete analysis of these structures. 

 All the more do I regret that, in opposition to the former correct opinion held by von 

 Siebold, van Beneden, and myself (" Blasenbandwurmer," p. 79), I unfortunately repre- 

 sented the yolk-gland as the ovary, and described the real germ-gland as the yolk-gland. 

 To any one who knows the difficulties which the investigator encounters at this very part, 

 the mistake will appeat pardonable. It is all the more easily committed, since the con- 

 tents of the two glands have often a great resemblance. Further, later observers, and 

 particularly Stieda, Feuereisen, and v. Linstow, have made the same mistake. 



