m\iir.n\T OK 'nil-: TWO 



i: <;KHM I.AVKI;S 



101) 



the processes in . \mphioxus, Amphihia, Selachians, IJird -, and Mam 

 mills, since they diller somewhat from one another. 



Tin' Itisten/ ef tin' </Vn/< >/>nit'//f <>/ A /// />// it'.rns Li nc,\>lnt its is \er\' ill 

 -Iriiclixo. The "aslrula elon.";ales, whereby t he Cteloilt eron is turned 

 a hi I le towards the hit nro dorsal surface, ami hero terminates in the 

 Ma .loporo, which marks ( ho future hind end of (ho worm shaped 

 d\ . Then ( he dorsal snrlace hecomos somewhat flattened ; i ho 

 Is iu tins region increase in height, become cylindrical, and lorm 

 10 medullary or neural plate (li". li!> ni/>). l>\ a h".hl infoldinv, of 

 ic laller, there arises a. medullary i^roo\e, which forces downward 



oof of the 

 lenleron in 

 ic form of a, 



(</,). At 



place \\ here 

 ie ihickened 



d u I I a r \ 

 lie joins (he 



1 a I I celled 

 rt of t, he 



iter germ- I|f (m o,,u,.ii..Kit...i..mii, 



i\er, or I he Ainphioxua with flvo pninit 



... I. XI.I.-.IMI //. |,,,:,l,-ii,.r ,-n.l : ./.. MIII.-I. ,-., I. II.- ,ron I. i\.-r ;,//. 



W ('*") 1 llll.v,lili:il . M. i ..ll . '. In I 



u 1 1 er ru | >l ion 1. 1 mi it m- Hfgmwnt; iwA. ...\ it y >!' i>umiti\,' -. r .m< m 



I he conl inn 



no\\ lakes place, and the epulermr. ;-ro\\ oxer I he ciirxed 

 mal plale from holh nlc . null) il halve, im el in (he middle 

 e and fuse Thir- lliere arises aloii- I h- hack of (he embryo 

 i" <(>) a canal, I he lo\\er wall ol \\hlcli I. loi'llied l>\ (he cill'Ved 

 iedullar\ plale (////'). .'Mid I he upper \\.-ill l>\ the o\ or^rowin^ epi 

 (,//). Il is onl\ al a la I ei s( a-jo I hat I lie medulla r\ plale in 

 I v i ni^ under the epidormi , i i eon \crletl ml > a nem a I I nbe 

 J. 7'J n) IN I he I.endin." up of its ed^os an. I I hen fusion. As (Jie 

 lit of lhonor\oii:, s\slem hec.mii . dill'erent ialed, il extends 

 lai toward the posterior end of the embryo, that the hlastoporo, 

 n. -I, i located (here. III! tall vv 1 1 1 1 1 1 1 it lenilor\, a lid \\ith t ho 

 ure o| the neural lube is included W'iUlill the end of I he laller 

 Ih: manner il oecm . Ihal neural lulie and infest ina I lube, a:; 

 tOWAKKVSKY lirst obsM-\ed, are now, by means of lh<> bla lopore, 

 linuilv (li". (i.Sc;/)al I he po: I erior end of I he bod y. Tll<^ two 

 ier const it nle a canal c posed of I v\ o arms, I ho form of \\ Inch 



ugh an mbryo of 



i \ , 



