REPRODUCTION BY SPORES. SEXUAL REPRODUCTION. 



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layers are repeated in the commencing buds, and from them the 

 organs are differentiated. 



The reproduction by spores is characterised by the production 

 within the organism of cells, which develop into new individuals in 

 situ or after leaving the organism. But this conception of spores, 

 which is taken from the vegetable kingdom, can only be applied to 

 the Protozoa and coincides with endogenous cell-division. The cases 

 of so-called spore-formation amongst the Metazoa (germinal sacs of 

 Trematodes) are probably identical with egg formation, and are to be 

 reduced to a precocious maturation and spontaneous development of 

 ova (Parthenogenesis, Paedogenesis). 



The digenous or sexual reproduction depends upon the production 

 of two kinds of germinal cells, the combined action of which is 

 necessary for the de- 

 velopment of a new or- 

 ganism. The one form of 

 germ cells contains the 

 material from which the 

 new individual arises, and 

 is known as the egg -cell, 

 or merely egg (ovum). 

 The second form, the 

 sper m-c ell (spermato- 

 zoon), contains the ferti- 

 lising material, semen or 

 sperm, which fuses with 

 the contents of the egg- 

 cell, and in a way which 

 is not understood gives 

 the impetus to the de- 

 velopment of the egg. The cell structures from which the eggs and 

 sperm arise are called sexual organs, for reasons which will be evi- 

 dent in the sequel ; the eggs being produced in the female organ or 

 ovary, and the semen in the male organ or testis. The egg is the 

 female, and the semen the male product. 



The structure of the sexual organs presents extraordinary diffe- 

 rences and numerous grades of progressive complication. In the 

 simplest cases, both products arise in the body wall, the cells of which 

 give rise at determined places to ova or spermatozoa (Coelenterata). 

 Sometimes they arise in the ectoderm (Hydroid-Medusse), sometimes 

 in the entoderm (Acalepha, Anthozoa). A similar arrangement 



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Rs 



FIG. 90. Generative organs of a Heteropod (Pterotra- 

 chea) after R. Leuckart. a, Male-organs ; T, testis 

 Vd, vas deferens. b, female organs ; Ov, ovary ; EJ, 

 albumen gland ; Us, receptaculum seminis ; Va, va- 

 gina. 



