SEXUAL DEVELOPMENT OF THE PROGLOTTIDES. 447 



formation of the generative openings. In joints about 4*5 mm. broad 

 by 1 mm. long, which begin at about the fifth hundred, and are about 

 20 cm. distant from the head, one notices at the 

 end of the genital ducts a discoid swelling, which 

 encloses a shallow depression, and by its con- 

 nection with the vas deferens and vagina is seen 

 to be the genital papilla. The adjacent section 

 of the vas deferens exhibits a long thickening, 

 the first imperfect rudiment of the cirrhus- FIG. 253. Develop- 

 pouch. The coils of the vas deferens are hardly ent of the germ-produc- 



t/ ing organs. (x5.) 



perceptible, but the posterior end already 

 appears free, and without connection with the uterus, to which the 

 vagina is united by its terminal club-shaped swelling, both anteriorly 

 and posteriorly. Further, the uterus is now more sharply defined, is 

 more slender than before, and is usually prominent opposite the 

 generative ducts. 



The first traces of the germ-producing organs are to be found about 

 the four hundredth joint. They are first indicated by the hitherto 

 almost homogeneous and transparent parenchyma assuming a more 

 granular appearance. The groups of cells of which this consists 

 are at first but small, and of the same appearance throughout, but 

 differently arranged in the upper and lower halves of the joints, 

 so that one can thus early distinguish the male and female organs. 

 In the following joints this distinction becomes more marked. The 

 granules of the upper half become larger, and are distributed with 

 tolerable uniformity over the whole surface of the middle region, 

 while those in the lower half remain smaller, and become ever more 

 distinctly arranged, so as to form distinct organs. In this way two 

 plate-like masses arise, which enclose the lower end of the vagina and 

 uterus between them, while very soon a third streak appears near the 

 hinder margin of the joint. These three masses of 

 course represent the two ovaries and the yolk-gland, 

 while the granules of the anterior half are destined to 

 become the testicular vesicles. 



The joints in which one first sees the above 

 diferentiation distinctly are about the 420th behind 

 the head. In these one sees also for the first time that 

 the conducting apparatus are distinct canals, and Fl( *-254 Lower 



,.,.. , , -11 en< * f the vagina, 



not solid threads. The vas deferens has a coiled Bowing its con- 

 course, and the posterior end of the vagina also nection with the 



, ., . uterus, (x 30.) 



exhibits its subsequent structure. One can recognise 



already in the former terminal club (Fig. 254) a differentiation 



into seminal vesicle and shell-gland, which for a time lie one close 



