DEVELOPMENT. 



107 



reproductive cells which resemble ova in their method of origin, 

 and at once develop into larvae. The pseudovary is clearly derived 

 from the rudiment of the sexual gland, but without ever reaching 

 complete development (fig. 100). The ovary acquires to a certain 

 extent the signification of an organ for producing germ-cells, and 

 it is not improbable that many products (Redia, Sporocyst] 

 regarded as spores or germ-cells correspond to embryonic ovaries 

 which produce ova cirable of spontaneous development. 



FIG. 101. Ovum of Nephelis (after O. Hertwig) . a, the ovum half-an-hour after deposition. 

 a projection of the protoplasm indicates the commencing formation of the first polar body ; 

 the nuclear spindle is visible. 6, The same an hour later, with polar body extruded, and 

 after entrance of the spermatozoon. Sk, male pronucleus. c, The same another hour 

 later without egg membrane, and with two polar bodies and male pronucleus (Sk) d, the 

 same an hour later with approximated female and male pronucleij Bk, pular bodies. 



DEVELOPMENT. 



It follows from the facts of sexual reproduction that the simple 

 cell must be regarded as the starting-point for the development of 

 the organism. The contents of the ovum spontaneously or under 

 the influence of fertilization enter upon a series of changes, the final 

 result of which is the rudiment of the body of the embryo. These 

 changes consist essentially in a process of cell division which implicates 

 the whole protoplasm of the ovum, and is known as segmentation. 



