< BORIO-ALLANTOIC GRAFTS. 305 



In the adult bird it form- the large corpora bigemina which give 

 off ii" peripheral nerve-. 



In the embryo, at about the forty-ei-ht hour stage, the brain 

 wall in this region i- -tilticiently independent of the ectoderm to 

 ble the i-olation of thi- part of the brain with very little or no 

 mechanical injury. It is comparatively :vmove all of the 



meteiicephalon and the diencephalon from Mich pieces, so that 

 the Iran-plant.- contain only mesencephalon. Since Mich 

 material \ ield- ^r.ift - fa \orable for this study and M'IUV the injury 

 at an earlier levelopment is of ile e ity much greater, 



the entire 'i-riment- with this priniordium wa- n 



with t i --ue from chii k t hours of incubation. 



I or the -ake of i oinpari-on, a brief description of the ine-eti- 

 i i ph. ilon of tin- ei^ht-day chick will be given here. The whole 



in i- di\ ided into \\\,< portions, a basal and a tect,:l. T 

 Mirroimd three < onnei ted . a\ ities the larger one- bein^ tin- u\- 



md the -in, die' the aqueduct of Sylviu-, which is 



\\ it hin t lie IM-. 1 port ion of -the region. The differentiation of the 

 wall of the (oipoia bigemina, the tectum lobi optici, i- \erv 

 del:- It \\e exami- -ectioil of part of thi- region, we 



find that tin: -rtical layers presti > . T 



will be de-'i ribed in tlu- order in which they are toiind on exami- 

 nation in the ependyma to the periphery. 

 Next the UK an ependyma which is densely nucleated 

 .md fairly \\ide. IP 'in it, cell bodies appear to be mi. 

 toward the per:: The next layer is an inner liber /one 

 which -tand- in j trast to the nu- ; /one ju-t nieii- 

 tioned. It i- i.itlu-r wide and contains cord- <ompo-ed of the 

 mui a ting e!l bodie- and their processes. The mi-ration of t! 

 cell bodie- end- in a nuclear layer which lie- just peripherad to the 

 inner fiber la\ i r. The processes of the-e cell- extend -till further 

 out, formin- an outer fiber layer which i- much narrower than the 

 inner liber layer and < -mtains no nuclear -trand-. Bounding the 

 outer liber la\ er i- a membrane, the external limiting membrane. 

 ontheouter -ideof which is a thin layer of mesenchyme contain- 

 ing blond vessels. 



The outer liber layer contain- processes of thi <-ll bodie- of the 

 outer nuclear /one. The-e are -hown very clearly in tangential 



