MULTICELLULAR ANIMALS 197 



cephalothorax. The genital opening in both sexes is on the ventral 

 posterior region of the cephalothorax (the unit consisting of the 

 head and thorax) and in the male the appendages of the adjacent 

 anterior abdominal segments are modified to form the intromittent 

 organ. In members of the class Insecta the gonads and ducts are 

 situated in the abdomen and the genital opening is at the extreme 

 posterior end of the abdomen. In many species of insects the ap- 

 pendages of the posterior segment are modified to form a stout or 

 complex instrument for depositing the eggs. Obviously, among 

 invertebrates there is no apparent relationship in the arrangement 

 of the reproductive organs, nor do any of them appear to be the 

 basic type from which the reproductive systems of vertebrates may 

 have been derived. 



The Vertebrate Reproductive System. Among the verte- 

 brates the reproductive systems all represent modifications of a 

 single basic plan, which is best described by diagrams (Fig. 135). 

 The primary female organs, the ovaries and oviducts, are situated 

 in the peritoneal region of the coelom. The ova, when mature, are 

 shed directly into the coelomic cavity, and the oviducts open into 

 this cavity to receive them prior to fertilization. In mammals the 

 ovaries are closely associated with the internal open ends of the 

 oviducts; their situation reduces the probability of escape of the 

 ova into the peritoneal cavity. The outer ends of the oviducts form 

 the uterus, which may thus be double, or single, depending on the 

 extent of fusion of the ducts. The mesonephros is always incor- 

 porated into the male reproductive system. For example, in the 

 frog the mesonephric duct in the male serves to carry away both 

 the wastes from the mesonephros and the spermatozoa from the 

 testes. In the higher animals the excretory function of the meso- 

 nephric duct is dispensed with; the mesonephric tubules of the an- 

 terior region are retained as ducts that collect spermatozoa and the 

 mesonephric duct serves as the main sperm duct. In mammals the 

 gonads migrate posteriorly during development, but still retain 



