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TABLE XLIII. 



SYSTEMATIC SURVEY OF THE JIOST IMPORTANT PERIODS IN THE PHYLOGENY 

 OF THE URINARY AND SEXUAL ORGANS OF MAN. 197 



XLIII. A. First main division: the sexual organs (G) and the urinary 

 organs (U) are distinct. (The sexual or genital system (G) and the 

 excretory or urinary system act independently of each other.) 



I. First Period : Genitals and Kidneys of Gastr&ads. 



G. Single, scattered cells of the entoderm change into egg-cells ; single, 

 scattered cells of the exoderm into sperm-cells. 



U. Special urinary organs are as yet wholly wanting. Secretion is 

 performed by the cells of the exoderm. 



II. Second Period : Genitals and Kidneys of Primitive Worms. 



G. The egg-cells of the entoderm gather into groups (ovary -plates) ; as 

 do the sperm-cells of the exoderm (testis-plates). 



U. A pair of simple pouch-like skin-glands (products of the skin-sensory 

 layer) develop into extremely simple kidney-canals (excretory organs of the 

 Flat-worms, Platelminthes). 



III. Third Period : Genitals and Kidneys of Scolecida. 



G. After the differentiation of the four secondary germ-layers is com. 

 plete, the egg-cells pass from the skin-sensory layer into the skin-fibrous 

 layer ; the sperm-cells also pass from the intestinal-glandular layer into the 

 intestinal-fibrous layer. 



TJ. After the formation of the coslom is completed, the blind inner ends 

 of the two kidney-canals (or "primitive kidney ducts") open into the body- 

 cavity (cceloma). 



IV. Fourth Period : Genitals and Kidneys of Chordonia. 



G. The groups of egg-cells (ovarial plates) and the groups of sperm-cells 

 (testes-plates) meet at the boundary between the endoccelar (the visceral 

 intestinal-fibrous layer of the coelom-epithelium) and the exocoelar (the 

 parietal skin-fibrous layer of the ccelom-epithelium), so as to form the 

 hermaphrodite glands. 



U. The primitive kidney ducts differentiate into an excretory and a 

 glandular part. 



V. Fifth Period : Genitals and Kidneys of Acrania. 



G. The sexes become distinct. In the female, only the ovary is de- 

 veloped ; in the male, only the testes. 



U. The primitive kidney ducts remain simple (atrophied in AmpTiioxus). 



