CHAP. IV.] 



THE CEREBRAL VESICLES. 



59 



An Embrto Chick of the fiest dat (about thirtt-six hours) viewed 

 fkom below as a transparent object. 



]il. Outline of the pellucid area. 



FB. The forebrain or first cerebral vesicle, projecting from the sides of which 

 are seen the optic vesicles op. A definite head is now constituted, tlie 

 backward limit of the somatopleure fold being indicated by the faint 

 line S. 0. Around the head are seen the two limbs of tlie amniotic head- 

 fuld : one, the true amnion a, closely enveloping the head, the other, the 

 false amnion a', at some distance from it. The head is seen to project 

 beyond the anterior limit of the pellucid area. 



The splanchnopleure fold extends as far back as sp. Along its diverging 

 limbs are seen the conspicuous venous roots or omphalo-mesaraic veins, uniting 

 to form the heart A, which continuing forward as the bulbus arteriosus ha, is 

 lost in the substance of head just in front of the somatopieure fold. 



Lying (in this position of the embryo) under the heart is seen the broad 

 foregut d, the wide crescentic opening into which at the hind limit of the 

 splanchnopleure fold is very conspicuous. Beneath the foregut are faintly seen 

 the hind brain HB. and higher up and more distinctly the mid brain MB. 

 These are not yet completely differentiated, and their limits are in consequence 

 very obscurely indicated. 



