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ANATOMY OF VERTEBRATES. 



Stages of developement of the ovarian egg of a verte- 

 brate animal (continued), txxxvi. 



1o a scries of self-splittings of the impregnated centre, which 



' fissiparous' progeny assimi- 

 late or incorporate more or 

 less of the yolk. In fig. 4, a, 

 d is the impregnated germ- 

 yolk ; e the fluid between it 

 and the zona, f; f is albumen 

 from which the chorion, cho, 

 arises. In B, fig. 4, is shown 

 the first division or seamen- 

 tation of the germ-yolk ; c 

 shows the second division; and 

 D, a later stage in which the 

 properties of the impregnated 

 centre have been diffused 

 and distributed by fissiparous 

 multiplication amongst the countless nucleated cells which form 

 the germ-mass. 



Thus far the vertebrate germ resembles in form, structure, and 

 2 behaviour, the infusorial monad and the germ- 



stage of invertebrates. The next step impresses 

 upon the nascent being its c vertebrate ' type. 

 Linear rows of the nucleated cells coalesce and 

 become converted into the nervous axis, which 

 under the form or appearance of a double chord, 

 fig. 5, ch, marks the dorsal or ' neural ' aspect 

 sperm-ceii, with thr.c of the embryonal rudiment. The nutritive organs 



spermatoa, and their „ , . .. A1 1 .° 



nucleus the 'spennato- grow Irom the opposite side. Along the mter- 



zoou' (Cock). CLXXTII. . 1 • "I ,1 l • c ,1 11, 



space is laid the basis oi the skeleton, as a 

 gelatinous cylinder, in a membranous sheath, called e notochord,' l 

 3 which developes a pair of plates l neurad ' 2 



to enclose the nervous axis, and a pair of 

 plates ' hremad ' 3 to enclose the vascular 

 axis and organs of vegetati ve life. Flesh 

 and skin coextend with the enclosing plates. 

 This formation of two distinct parallel 

 cavities — s neural ' and ' hremal ' — under 

 symmetrical guidance in the vertical or 

 e neuro-haemal' direction, with a repeti- 

 tion of parts on the right and left sides, 

 ! establishing transverse or < bi-lateral ' 



1 The ' chorda dorsalis' of embiyologists. 2 Backward in man, upward in beasts., 



3 Forward in man, downward in beasts. 



