Tin: ..i:;\xa OF mi: 01 IKK ,KIC.M LATEH 



491 





Tin- memhranmix I 



. layer. H..\\. \er _M. ;|| is, a 



complication that : me l:th\: 



fundament i> exceedingly simple. It arises on tin- d.r>al Mil-fare of 



tin* emhr\o in tin- NgiOD of t In- medulla ohlnn. 



thelir-t \ i-. I :.! cleft all- 1 I In- Ht taelnin-nt it'the>.- vral arch 



(ti^r. L'7 1 alo\e the mi Here . 



outer ^'erin-laver Uvoine- thickened ar a into an 



pit. This process ran ) trao-1 \<M-\ fa>ily in the .-ii: 

 Chick on and at't.-r tin- md oi ih-- B600IUJ day ..f ii 

 in tin- -inl)i-yo Rabbit tit'i- 



11. Th'- :nnliir\ n i-\- 

 makes it> way from tln> hraii:. 

 near at hand, to tin- t'mnlu.- 

 of the pit, \\licreit terminal. > 

 in a ganglionic nlar^oin.-nt . 



Tin- IJony Kisht-s al..i. 

 liiliit a il-\iati;n from 

 n unlit ion>. Just as the r> 

 nervous s}'stem was in their 

 case formed not as a tuhe. hut 



Milid Ixxly. and tl 

 not as a Ve-irle. hut 

 epitliflial hall. SO WO 8C'i 

 that- in>tead of an audit* 

 pit then . n> 



of the proliferation of the id epithelial j 



This, like the brain-tubr and tin- eye-vesicle, acquire- :;n internal 

 c-luuuher : 



The next stage shows the pit CMii\eiied int.. 

 hi the Chick this take- place in the course of tin- third -i 

 inva^inati.'n of the outer germ-layer grows deeper and d. ep r. ami 

 l.y tho approximation of itfl margins becomes pear-shaped ; >oon the 

 connection wit h the outer germ layer becomes entirely lost, as is sh 

 l>v a sH-tiin tliroui:h the h. -iieep (fig. 275 Ib). 



In nearl\ all Vei t.-hrates the auditory vesicle is constricted off 



tVolll tile e, Model-Ill ill the -allie ll. I'lie Selaclliall- a! 



ption : here th- auditory \e>ii-le which is metamorphosed into the 

 laltyrinth retains pern. 'inection with the surface oi 



Tig. 274. Head of a human embryo 75 mm. long, 

 nock measurement. 'Menachlicbr 



K'nl'l '. i -lu-li.' 



-siele lie* above the fint ruoerai 



In tin- iit,nmf-r?uee of the visceral 



there are to be teen six eleraiiona, de- 



rifMted t.% wu ihtfk a vft h fcbi 



