532 EMBRYOLOGY 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 l>e noticed how away on each side it shows the normal four layers 

 of cells ectoderm, somatic mesoderm, splanchnic rnesoderm, 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. 



o.r 



FIG. 234D. Transverse section of a second-day Fowl eml>ry<> passing through 

 the optic rudiments. 



ect, ectoderm ; tntl, endrxlerm ; ntvs, inescm-liyine ; o.r, optic rudiment ; 

 *///'. tplanchnoooele ; thai, roof of thalamencephalon. 



. proamnion 



This two -layered region of blastoderm is the proamuion before 

 alluded to. 



The head itself is occupied almost entirely by the brain rudiment 

 the thalamencephalon in the centre (thai) continued outwards >n 

 each side as tin- 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 com UK MIC ing to recede from the latter, the space 

 between the two lu-in- occupied by mesenchvme (mes). 



TIIK THII:I> I)AV OF Ivri'.ATinx. 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 mo>t iui.-n-st ing of all to the morphol<,-ist. K,.r ih,> 

 student \\ho is training himself practically in the technique of 

 embryolo'jiral observation there is no liner malt-rial than that 

 afforded by I'.ird embryos of about ih. far lea ruing one of tin- 



most important parts of that technique namely the interpretation 

 of serial sections. 



