IV.] 



THE ALIMENTARY CANAL. 

 FIG. 15. 



61 



EMBRYO OF THE CHICK AT 36 HOURS VIEWED FROM ABOVE AS AN OPAQUE 



OBJECT. (Chromic acid preparation.) 

 /. b. front brain, m. b. mid brain. Ti. b. hind brain, op. v. optic vesicle. 



au. p. auditory vesicle, o. f. omphalo-mesaraic vein. p. v. protovertebra. 



m. f. line of junction of the medullary folds above the medullary canal. 



8. r. sinus rhomboidalis. t. tail-fold, p. r. remains of primitive groove. 



a. p. area pellucida. 



The line to the side between p. v. and m. f. represents the true length 

 of the embryo. 



The biscuit-shaped outline indicates the margin of the pellucid area. The 

 head, which reaches as far back as o.f, is distinctly marked off ; but neither the 

 somatopleuric nor splanchnopleuric folds are shewn in the figure ; the latter 

 diverge at the level of o.f, the former considerably nearer the front, somewhere 

 between the lines m. b. and h. b. The optic vesicles op. v. are seen bulging 

 out beneath the superficial epiblast. The heart lying underneath the opaque 

 body cannot be seen. The tail-fold, t., is just indicated ; no distinct lateral 

 folds are as yet visible in the region midway between head and tail. At m. /. 

 the line of junction between the medullary folds is still visible, being lost 

 forwards over the cerebral vesicles, while behind the folds diverge to enclose 

 the narrowing sinus rhomboidalis, s. r. 



