BY JAvS. P. HILL AND C. J. MARTIN. 47 



backwards. The fourth pair are much less distinct than the 

 others, and in surface view are not so sharply limited from the 

 medullary plate. 



In the anterior region of the mid-])rain, a pair of neuromeres 

 (PL IX., N.) occur, ])ut they are much less distinct than those of the 

 hind-brain, appearing merely as local thickenings of the medullary 

 plate in that region, without any accompanying constrictions. 



Opposite the second and third neuromeres of the hind-brain 

 there is on each side a somewhat triangular thickened patch of 

 ectoderm — the auditory plate (PI. ix., aud.). The mesoderm in 

 the region of the auditory plates is very thin, hence they stand 

 out very distinctly. The anterior margin of each plate is on a 

 level with the front edge of the second neuromere; indeed the 

 greater portion of the plate is situated opposite this neuromere. 

 Each plate is roughly triangular in shape, with the base of the 

 triangle adjoining the medullary plate, and with a deep bay in 

 the middle of its posterior margin. 



The hind-brain region narrows gradually posteriorly and passes 

 into the medullary plate of the future spinal cord. The medullar}^ 

 plate widens out towards the posterior end of the embryo into a 

 well marked sinus rhomboidalis which completely surrounds the 

 primitive streak. The primitive streak is just visible in the photo- 

 micrograph (PL IX., pr. s.) as a faint linear thickening enclosing 

 a whitish axial line — the primitive groove — at the posterior 

 end of the embryo. The anterior end of the primitive streak 

 exhibits a distinct thickening, to one side of which the blastopore 

 (PL IX., hi.) is situated. This thickening is continued forwards 

 as the head process of the mesoderm which passes into the hinder 

 end of the notochord. The notochord is very distinct in the 

 photo-micrograph as the longitudinal line running along the 

 middle of the medullary plate. At its anterior end it broadens 

 out, and terminates about the middle of the future fore-brain. 



The bilateral Anlagen of the heart are very noticeable in surface 

 view (PL IX., /t.a.) as two tubular-looking structures lying external 

 to the auditory plates, and extending backwards from them along 

 the outer edges of the forward extensions of the lateral zones of 



