ANATOMY OF VERTEBRATES. 



stages of dcveloperaent of the ovarian ctrfe' of a verto- 

 brate animal {continued), clxxvi. 



to a series of self-si^littings of the impregnated centre, whicli 



' fissiparous' j)rogcny 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 segmen- 

 tation of the germ-yolk ; c 

 shows the second division; and 

 D, a later stage in which the 

 '■^z^'c^c'^^il:^^ properties of the impregnated 



centre have been diffused 

 and distributed by fissiparous 

 midtiplication 'amongst the countless nucleated cells which form 

 the eerm-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 ' 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, cli, marks the dorsal or ' neural ' aspect 

 of the embryonal rudiment. The nutritive organs 

 grow from the opposite side. Along the inter- 

 space is laid the basis of the skeleton, as a 

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

 3 which de velopes a pair of jjlates ' neurad ' ^ 



to enclose the nervous axis, and a pair of 

 plates ^ hfemad ' ' to enclose the vascular 

 axis and organs of vegetative life. Flesh 

 and skin coextend with the enclosing plates. 

 This formation of two distinct parallel 

 cavities — ' neural ' and ' hremal ' — under 

 symmetrical guidance in the vertical or 

 ' neuro-hannal' direction, with a repeti- 

 tion of parts on the right and left sides, 

 establishing transverse or 'bi-latcral' 



Spcnii-cclJ, with tlireo 

 spei-matoa, and tinii- 

 nucleus tlie 'spLTmato- 

 zoon' (Cock). CLXivii. 



^Clt 1 



The ' chunia dorsaiis' of cinbvyologists. - Bi)ck\varJ in man, ujnvmd in beasts. 



" Foi\Viii'd ill man, dowi:\varJ in beasts. 



