ANATOMY OF VERTEBKATES. 



to a series 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 ; c the fluid between it 

 and the zona,^/; 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 

 properties of the impregnated 



Stages of developement of the ovarian egg of a verte- haV6 



brate animal (cowled). CLXXVI. 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 ' 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 f neural ' aspect 

 with three of the embryonal rudiment. The nutritive organs 



spermatoa, and their . . ., ., , i 



nucleus the 'spermato- grow irom the opposite side. Along the mter- 



zoon ' (Cock). CLXXVII. i i j i i P-I 11 



space is laid the basis ot the skeleton, as a 

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

 3 which developes a pair of plates ( neurad ' 2 



to enclose the nervous axis, and a pair of 

 plates ( haamad ' 3 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 e haemal ' -under 

 symmetrical guidance in the vertical or 

 { neuro-haamal ' direction, with a repeti- 

 tion of parts 011 the right and left sides, 

 transverse or ( bi-lateral ' 



I 



Vertebrate egg, impregnated by the 

 spermatozoa (Rabbit). CLXXVI. 



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



3 Forward in man, downward in beasts. 



