532 EMBEYOLOGY OF THE LOWER VERTEBRATES ch. 



passes outwards and upwards round the foregut to become continuous 

 with the dorsal aorta. These two hoop-like vessels which connect up 

 ventral and dorsal aortae are the first pair of aortic arches. 



Still further forward the region of the forebrain and optic 

 rudiments is reached (Fig. 234d). 



Owing to the folding off of the head rudiment the section of the 

 head itself appears completely detached from the blastoderm and the 

 latter is beginning to form a depression which will later become 

 more marked and in which the head will lie. In the blastoderm it 

 will be noticed how away on each side it shows the normal four layers 

 of cells — ectoderm, somatic mesoderm, splanchnic mesoderm, endo- 

 derm — while on the other hand in the region underlying the head of 

 the embryo it is only two layered the mesoderm being here absent. 



Fig. 234d. — Transverse section of a second-day Fowl embryo passing through 

 the optic rudiments. 



ect, ectoderm ; end, endoderm ; ?>ies, mesenchyme ; o.r, optic rudiment ; pa t proamnion ; 

 splc, splanchnocoele ; thal,'ioof of thalamencephalon. 



This two -layered region of blastoderm is the proamnion before 

 alluded to. 



The head itself is occupied almost entirely by the brain rudiment 

 — the thalamencephalon in the centre (thai) continued outwards on 

 each side as the optic rudiment (o.r). For the most part the external 

 ectoderm is closely apposed to the surface of the brain but dorsally 

 the former is commencing to recede from the latter, the space 

 between the two being occupied by mesenchyme (mes). 



The Third Day of Incubation. — During the later hours of the 

 second and earlier hours of the third day of incubation there take 

 place a number of important changes which render this period 

 perhaps the most interesting of all to the morphologist. For the 

 student who is training himself practically in the technique of 

 embryological observation there is no finer material than that 

 afforded by Bird embryos of about this age for learning one of the 

 most important parts of that technique namely the interpretation 

 of serial sections. 



