IV.] THE BRAIN. 79 



medullary canal ; s.r. sinus rhomboidalis ; t. tail-fold ; p.r. 

 remains of primitive groove (not satisfactorily represented) ; 

 a.p. area pellucida. 



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

 length of the embryo. 



The fiddle-shaped outline indicates the margin of the pellucid 

 area. The head, which reaches as far back as o.f., is dis- 

 tinctly marked off; but neither the somatopleuric nor 

 splanchnopleuric folds are shewn in the figure ; the latter 

 diverge at the level of o./., 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 superfi- 

 cial epiblast. The heart lying underneath the opaque body 

 cannot be seen. The tail-fold t. is just indicated ; no dis- 

 tinct lateral folds are as yet visible in the region midway 

 between head and tail. At m.f. the line of junction between 

 the medullary folds is K still visible, being lost forwards over 

 the cerebral vesicles, while behind may be seen the remains 

 of the sinus rhomboidalis, s.r. 



medullary folds coalesce completely in the cephalic 

 region, the front end of the neural canal dilates into 

 a small bulb, whose cavity remains continuous with 

 the rest of the canal, and whose walls are similarly 

 formed of epiblast. This bulb is known as the first 

 cerebral vesicle, Fig. 27, /A, and makes its appearance 

 in the early hours of the second day. From its sides 

 two lateral processes almost at once grow out : they are 

 known as the optic vesicles (Fig. 27, op. v.), and their 

 history will be dealt with at length somewhat later. 

 Behind the first cerebral vesicle a second and a third 

 soon make their appearance ; they are successively 

 formed very shortly after the first vesicle ; but the 

 consideration of them may be conveniently reserved to- 

 a later period. At the level of the hind end of the 



