CHORIO-ALLAXTOIC GRAFTS. 309 



fiber layer is reduced or absent ; processes of the nerve cells 

 penetrate the mesenchyme, forming nerve-like structure: 5 ; and 

 the layers of the cortex are very heavily vasculari/ed. 



This graft shows a single large mesocce!' < - the other -< 



de-cribed. At one end of the sections of both graft-, where the 

 roof of the mesencephalon is cut tangentially. -ive fiber 



tracts are visible in the inner fiber layer. The mesoccele in both 

 case- ; - iompletely eiicl..-ed by nervou- ii--ue, the brain having 

 fu-fd .it both ends. This was aided in all probability by the 

 apposition of the cu --tion. T ; - s morphology of 



the organ ari-ing fn 'in -u< h a ^raft seems theret'i >re to i . led 



io ijiiiit: a con-ideral'' it by the en\ ironmeiit though the 



differentiation of it- cellular constituents is specific. 



The growth and differentiation of the mesencephalon i 



!e ii|)on the chorio-.ill.mtoic membrane of the chick embr\o 



brings out a number of "N cry interesting far* s. Phe basal and the 



iei lal portion- retain their individuality though the two cavities 



Mended into .me large nie-i > ( ofle. The ba-al port ion ( le\ el"| is 



more or U- UK ^ularK . dependent apparently on the mechanical 



in\ol\ed .UK! tin- ab-ence of ascend: .-nding 



L- when i-Maicd. T : .il portion, ho\ve\er, -ho\\ - two 



type- of reaction, both of which have already been dc-cribid. 



The iniere-i:: about the development of tin- part i- toiind 



in the form taken b\ the outer liber la; :dent on the 



presence or absence "I t' rnal limiting membrane. This 



a !- tS the other la\ er- illa-UUH 1. a- thi- membrane appt 



to regulate the peiiet rat ion <>t blood from the host 



Mipi . Ph< growth and the differentiation of the various lay- 

 ers i- very -pctilic in both < I he nuclear cords and the 

 \\idth of the layer-, \\ith the e.\cepti<'ii of the outer fiber layer, 

 as ha- been mentioned above, are appro\imati-I\- normal. 



It was mentioned above that in >ome of the graft- of the 

 mesencephalon and the na-al re-ion, thi-re are -mall peripheral 

 nerve-like bundle- pre-ent which do not correspond to any Mich 

 formation in the normal. The-e outgrowths of ner\'e are the 

 n-ult of the differentiation of the individual liber- of the nerve 

 cell-, but, owing to the fact that the membrane which furni-hes 

 a mechanical ob-tructioii to indefinite outward growth of 



