THE UROGENITAL SYSTEM 



297 



that of mammals makes its first appearance in Amphibia as a hollow out- 

 growth from the floor of the cloaca. In neither the amphibians nor 

 reptiles, however, is the bladder directly connected with the excretory 

 ducts, so that excretions reach it only by way of the cloaca. In those 



ECTODERM 



CIRCULAR 

 MUSCLES 



DORS NERVE-CORD 

 LONG. MUSCLES 



PROBOSCIS SHEATH 

 k- PROBOSCIS 



GONAD 



LATERAL 

 -NERVE-CORD 



LATERAL 

 -BLOOD-VESSEL 



PARENCHYMA 



NEMERTEAN-CROSS SECTION 



Fig. 267. — A cross section of a nemertean. While the nemertean lacks a coelom, 

 the gonadic sacs have been compared with the coelomic sacs of coelomate animals. 

 (Redrawn after Lang.) 



mammals which are without a cloaca, the ureters acquire direct connexions 

 with the bladder and open upon its dorsal surface. 



B. Phylogenesis of the Reproductive System 



Cells specialized as reproductive elements make their first appearance 

 in colonial Protozoa. Sexual reproduction, however, involving the union 

 of two gametes occurs in all classes of Protozoa. Many Protozoa conju- 

 gate periodically. The beginnings of differentiation of gametes appear 

 also in Protozoa. In unicellular organisms transitional stages between 

 the union of similar gametes, isogamy, and that of speciaUzed eggs and 

 spermatozoa, heterogamy, may be recognized. Such a differentiation of 

 gametes is generally interpreted as adaptive. The ovum contains the 

 food supply for the developing embryo and consequently loses the motility 

 which is retained by the spermatozoon as a way of insuring union of 

 gametes. Biologists are not agreed in regard to the meaning of sexual 

 reproduction. Some hold that it increases variability, while others assume 

 that it increases stabiUty of species by checking extreme variation. 



