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produces an intermediate form, so that the offspring is not 

 like its parent, but its grandparent. Thus the jelly fish or 

 sea nettle is first a ciliated germ, then a hydra-like polyp, 

 which produces colonies of medusa? by division or budding. 

 These become free, enlarge, and propagate germs. 



(4) Parthenogenesis, or virgin production, is another variety 

 of generation by which one or more broods may occur with- 

 out sexual union. The aphides, at the close of autumn, are 

 winged males and wingless females, whose ova lie dormant in 

 winter, but the young of summer produce ten or more sexless 

 generations, and a final brood as before. The eggs of male 

 or drone bees are said to be laid by unimpregnated bees, 

 queens, or even workers. Similar instances occur also in 

 other insects. Cases of fcetation by inclusion (an egg within 

 an egg), and other curiosities of embryology in higher ani- 

 mals, may be similar. 



(5) The development of the ovum in higher animals presents 

 an extensive field for study, but a few particulars will serve 

 our introduction to histological structure. The germ cell, or 

 ovum, is a globular epithelial cell of the Graafian vesicle of 

 the ovary. The latter is constituted by an infolding of the 

 surface so that the epithelial covering becomes the lining of 

 the vesicle. The ovum is a product of this epithelial lining, 

 and consists of a vitelline membrane, or outer surface, the 

 vitellus or yolk, the germinal vesicle or nucleus, and the 

 germinal spot or nucleolus. Fig. 4, PI. 2. The sperm cell, 

 or spermatozoan, is in like manner developed from the glan- 

 dular epithelium of the seminiferous tubules in the male. 

 After impregnation the self-division or segmentation of the 

 vitellus occurs, preceded by separation of the nuclear fibers 

 and other vital phenomena. 



In some animals the entire contents of the ovum participate 

 in the segmentation and development, but in vertebrates a 

 layer of cells is formed by subdivision of bioplasm on the sur- 

 face of the ovum. This occurs on one side, forming the 

 blastoderm, to which all subsequent processes are restricted. 

 The blastoderm separates into three layers, the epiblast, the 

 mesoblast, and the hypoblast. The outer one, or epiblast, 



