UROGENITAL SYSTEM 



227 



Man belongs to that group of metazoa in which the con- 

 junction of a male individual with a female is requisite for this 

 fertilisation. In the mode of origin of the ovum and its early 

 development the human species is identical with the animals 

 immediately below it in the biological .scale (Huxley). 



The impregnation of the ovum commences with the extru- 

 sion of the two so-called " directive bodies " or " polar cells " ; 

 the central particle of the penetrating spermatozoon replaces 

 the centrosome, or germinal vesicle, of the ovum, by com- 

 bining with the female pro-nucleus to form the first segmenta- 

 tion nucleus (archicaryon) ; after this blending has taken place, 

 segmentation of the ovum proceeds to the morula-stage, and 

 subsequently the embryonic vesicle is formed (blastula-stage) ; 

 by the invagination of the wall of the blastula at one point 

 the so-called gastrula-stage is reached (Figs. 1 16 and 1 17). 



FIG. 116. Gastrula of primitive-gut 

 animal or gastraead. 



FIG. 117. Gastrula of amphioxus. 

 (Haeckel, Anthropogenic.) 



The two original layers of cells of the gastrula are the 

 primary germinal-layers which contribute to the building of 

 the whole organism ; the outer stratum or skin-layer is the 

 ectoderm from which develop the external skin, the central 

 nervous system and the sense-organs ; the inner stratum, or 

 gut-layer, is the entoderm, from which the alimentary canal and 

 its appendages, including the lungs and kidneys, arise. Ulti- 

 mately a middle layer (mesoderm) develops between these 

 two layers, and from this mesoderm the skeleton, muscles, 

 fibrous and tegumentary covering of the body-wall and limbs, 



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