SEXUAL ORGANS AND TISSUES. 55 



reproductive ducts are also in various degrees associated with excre- 

 tory functions. For an account of the origin of the ducts in higher 

 animals, the reader must be referred to the embryological textbooks of 

 Balfour and Hertwig, or most conveniently of Haddon. Similarly for 

 such modifications as that of the female duct into oviduct and uterus, 

 reference must be made to the larger anatomical works of Gegenbaur 

 and Wiedersheim, or for a briefer account to Parkeris translation and 

 edition of Wiedersheim' s smaller textbook, and to Prof. Jeffrey Bell's 

 work already mentioned. 



III. Yelk-Glands. — As we shall afterwards see, the ovum is 

 often furnished with a large quantity of nutrient material. This 

 serves as the food-capital for the growing embryo or young larva. 

 It is obtained in various ways — from the vascular fluid, from the 

 sacrifice of adjacent cells, or from special organs known as yelk-glands 

 or vitellaria. The yelk-glands, as they occur for instance in some 

 of the lower worms (turbellarians, flukes, tapeworms,) are of some 

 general interest. They represent, as Graff has shown, a degenerate 

 portion of the ovary, in which the cells have become even more 

 highly nutritive than ova. "The origin of the yelk-gland," Gegen- 

 baur says, ' ' is probably to be found in the division of labor of a 

 primitively very large ovary." In more technical language, yelk- 

 glands are hypertophied or hyperanabolic portions of the ovary. 

 Apart from this nutritive capital, the egg is often equipped with 

 envelopes or shells of some sort, which may be furnished by special 

 •organs, or by the sacrifice of surrounding cells, or by the walls of 

 the ducts as the eggs pass out. 



IV. Organs Auxiliary to Impregnation. — In most animals 

 in which internal fertilization of the ova occurs, there are in both 

 sexes special structures auxiliary to the function of impregnation. 

 Thus the end of the male canal is commonly modified into an intro- 

 mittent tube or penis, through which the male elements flow into 

 the female duct. In the crustaceans some of the external appendages 

 are often modified, as in the crayfish, to serve this purpose, and the 

 same is the case with minute structures on the posterior abdomen 

 of many insects. Sometimes, as in the snail {Helix), which may 

 be taken as an extreme type of reproductive specialization, separate 

 organs are present, in which the spermatozoa are compacted into 

 masses or packets, known as spermatophores. In most cuttlefishes, 

 these pass from the male ducts to one of the "arms," which thus 

 laden is occasionally set free bodily into the mantle-cavity of the 

 female, where it was of old mistaken for a worm, and called Hecto- 

 cotylus. So in some spiders, the palps near the mouth receive the 

 male elements and transfer them to the female. Special storing 



