THE OEGANS. 53 



in certain Ccelenterata and Mollusca we may suppose that primi- 

 tively the relations of the two are very different, for in these forms 

 the ova are derived from the endoderm, and the sperm from the 

 ectoderm. The endoderm is, therefore, the female, and the ectoderm 

 the male germinal layer. But it is not yet known how far these 

 relations obtain among the higher animals. As yet there are only 

 uncertain indications, but these speak to a general agreement with 

 the results already obtained. 



The parts of the body which are set apart for the formation 

 of the sexual products gradually take on the form of glands. This 

 is a further step in differentiation, and is connected with the local- 

 isation of the function. 



In the simplest cases the two kinds of generative products are 

 formed in special parts of the body, which function as sexual organs 

 (reproductive glands) ; but these parts are not at first distinguished 

 by any special characters. The organs which produce the semen are 

 called testes, and those which produce ova, ovaries. Going a 

 step further, we find the reproductive glands still further differen- 

 tiated. In their simplest condition the products of these organs 

 merely break away from the spot where they are formed, and pass 

 into the digestive sac, or into the body-cavity, or even directly to 

 the exterior. Gradually, however, ducts, which are often very com- 

 plicated in character, are added on : it is probable that these ducts 

 are not primitively connected with the germinal glands. Where 

 these ducts can be seen to have any relations to other organs, these 

 appear to be excretory organs (§ 40) which have entered into the 

 service of the genital organs, and have been altered so as to 

 correspond to this function. It becomes a great question whether 

 the excretory ducts of the reproductive matter are not in all cases 

 excretory organs. Receptacles which serve for the collection of 

 the sperm are formed on the outlet-tubes (seminal ducts) of the 

 organs which produce the sperm ; from the wall of these canals 

 glands are differentiated, which secrete a fluid to be mixed with the 

 sperm ; finally, there are arrangements for passing the sperm into 

 the system of the other sex (copulatory organs). The differen- 

 tiations of the egg-forming organ are no less varied ; the duct 

 (oviduct) of the ovary is provided with dilatations, in which the 

 ova get special envelopes, or are further developed. These portions 

 of the oviduct are called the uterus. Special glands, Yolk 

 glands, are formed from the ovary, and secrete a substance which 

 is either taken up by the ovum or which merely forms an envelope 

 for it. Appended organs receive the semen which is passed in 

 copulation, and are known as receptacula seminis; lastly, other 

 parts serve for the reception of the copulatory organ, or for the 

 deposition or preservation of the ova. 



The relation of the egg-forming and sperm-forming organs to 

 one another varies greatly, and must be considered from the stand- 

 point of differentiation. In the lower divisions organs of both 

 kinds are united with one another, sometimes in such a way that one 



