SEXUAL ORGANS AND TISSUES. 6 1 



passage of spermatozoa and the exit of ova or embryos, it ought further to 

 be noticed that the ovaries can hardly ever be said to be in direct connec- 

 tion with their ducts. The ova usually burst from the ovary into the body- 

 cavity, whence they are more or less immediately caught up by, or forced 

 into the canals, by which they pass outwards. With the testes it is different, 

 for if ducts be present, they are in direct connection with the organs. 



It is enough to state that in the great majority of cases ducts are 

 associated with the essential organs. Those of the male serve for the exit 

 of the spermatozoa, and may be terminally modified as intromittent organs. 

 Those of the females serve either solely for the emission of unfertilised eggs, 

 or for the reception of spermatozoa, and the subsequent exit of fertilised 

 ova or growing embryos. In some worm-types, and in all vertebrates, 

 from amphibians onwards, the reproductive ducts are also in various degrees 

 associated with excretory functions. For an account of the origin of the 

 ducts in higher animals, the reader must be referred to the embryological 

 text-books of Balfour and Hertwig, or most conveniently of Haddon. 

 Similarly for such modifications as that of the female duct ir)to oviduct and 

 uterus, reference must be made to the larger anatomical works of Gegenbaur 

 and Wiedersheim, or for a briefer account to Parker's translation and edition 

 of Wiedersheim's smaller text-book, and to Prof. Jeffrey Bell's work already 

 mentioned. 



§ 3. Yolk-Glatids.—k.'i 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 

 yolk-glands or vitellaria. The yolk-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," Gegenbaur says, "is probably 

 to be found in the division of labour of a primitively very large 

 ovary." In more technical language, yolk-glands are hypertro- 

 phied or hyper-anabolic 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. 



§ 4. Orgafis Auxiliaiy to Impregnation. — In most animals 

 in which internal fertilisation of the ova occurs, there are in 

 both sexes special structures auxiliary to the function of impreg- 

 nation. Thus the end of the male canal is commonly modified 

 into an intromittent 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 cray- 



