>r:.n.N i OF THE i \v - MIHIM.I: 01:101 i 



109 



-,^-i- .-!-: ^~ 

 '- - 



the procee> in An,phi...\u>. Ain|.lii!.i:i. Selachian^ : Main- 



n. a U. >ince they ditl'er somewhat from one another. 



'/'/! Itistin-ji >/(/ ih'ri'lo/.i, ///*// /M-//- 'iw \& very in- 



structive, Tin- gaMrula elo:i^ ; ,< be c.i-l.Miter,,n i- n. 



a little to wards the future dorsal surface, ami here ten :i the 



hla-top,, re. which marks the i'uttin- liiml end of t he \\ - .mi -haped 



body. Then tin' dorsal surface becomes somewhat flatt-ned; the 



cells in this region increase in ln-i^ht, 1- iimlriral, ami lurm 

 the m.-dullai-y or nnnal plate (fi.ir. I'.'.) ////<). By a -li.L'ht int'.ldi! 



the latter, there arises a medullary which for, 



f of the 



ecdenteron in '* dk ttS ' u<A uth mk <* 

 the form of a 

 iih:o (c/<). At 



the J.lace \\here 

 the thickened 

 medullary 



j'late joins tin' 



small -celled 



{art of the 

 outer germ- 

 layer, or the 



horn-layer (A6), 



an interruption 

 in the continu- 

 ity now takes place, and the epidermis grows over the curve.! 

 neural plate from both sides, until its halves meet in the middle 

 line and l'u>e. Thus there arises along the back of the embryo 

 7") ; , canal, the lower wall of which is formed by the curved 

 medullary plate (m/i). and the upper wall by the overgrowing e pi - 

 der,ni> (<//,). It i> only at a later >tage that the medullary plate in 

 Amphioxu.x, lying under the pidermi.-. i> converted into a neural tube 

 (tig. 7'J //) )>y the luMiding up of it- e.l^e- and their t'u-ion. As tlu- 

 fui'.dainent of the nervous system Ixvom, -. differentiated, it extends 



a toward the posterior 'lie emhryo, that the l.la^topore, 



which is located there, still falls within it> b and with the 



clo-ure of the neural tube is indude.l within the lie latter 



In this manner it occurs that neural tuhe and inte>tinal tube, as 

 KONSAI. KVXKV first observed, are now, by mean** of the l.la-toj>ore, 

 in continuity (tig. 68 en) at the poterir end of the body. The two 

 together constitute a canal composed of two arms, the form of which 



Fig. 68. Optical longitudinal [sagittal] section through an embryo of 

 Amphiozus with five primitive segments, K. 



T, Ant.-ii..r. //. ]< 'He germ-layer; dh, 



inU'stiii:il cavity ; n, neural tube ; en, neurenteric canal ; u', first 



pin.. -,t ; uih, cavity of primitive scgn 



