. Chordate anatomy. Chordata; Anatomy, Comparative. 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 c
. Chordate anatomy. Chordata; Anatomy, Comparative. 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 Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemb
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