MORPHOLOGY OF ANIMALS 



posterior portion may enlarge to form a uterus. The sper- 

 miduct is the segmental duct itself (Fig. 21, C, sd) ; in 

 Amphibia it carries off both urine and spermatozoa (Fig. 

 21, C) ; in animals above the amphibians, which have a 

 metanephros and ureter, it acts exclusively as a spermi- 

 duct (Fig. 21, D). Some of the mesonephric tubules 

 (nephridia) grow into the testes and become the vasa 

 eferentia and epididymis (Fig. 21, D, ep). There is thus 

 an intimate relation between excretory ducts and genital 

 ducts, and therefore these two systems in vertebrates are 

 usually classed together as the urino-genital system. 



8. Reproductive System 



Reproduction among animals is both sexual and asexual ; 

 the former occurs among all animals, the latter is limited 

 to the lower forms and to the constituent cells and organs 

 of higher ones. 



Sexual Reproduction; Sexes. Sexual reproduction or 

 amphigony consists in the union of the nuclei of two cells, 

 the sex cells or gametes, to form a single nucleus or cell, 

 of double origin, the oosperm or zygote, from which a new 

 individual similar to the parental form develops. If the 

 gametes are approximately equal in form and size their 

 union is spoken of as conjugation, if they are very unlike 

 in these respects they are called ova and spermatozoa, 

 and their union is known as fertilization. Both conjuga- 

 tion and fertilization occur among the Protozoa, whereas 

 Metazoa reproduce by means of differentiated sex cells, 

 namely ova and spermatozoa. In a few animals ova have 

 the power of developing without previous fertilization, 

 the process being known as parthenogenesis. If such de- 



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