FOWL— SECOND DAY 



525 



than that of the ectoderm. This is brought out clearly by a sagittal 

 section such as that shown in Fig. 230. Such a section also brings 

 out the fact that while the greater part of the portion of blastoderm 

 tucked in beneath the head of the embryo is two-layered (proamnion), 

 there being no mesoderm present, this does not apply to the farthest 

 back part of the fold. Here, in the wide space between ectoderm and 

 endoderm, mesoderm has penetrated which will give rise to the peri- 

 cardiac wall and the heart. The medullary folds have met over a 

 considerable extent but still remain separate at their extreme front 

 ends as well as over the whole extent which will later form the 

 spinal cord. Here they bound a deep neural groove. Towards their 

 posterior ends the two medullary folds diverge to pass on either side 

 of a lance-shaped area (rhomboidal sinus) which they enclose by 

 converging towards one another behind it. Along the centre of the 



Fit;. 230. 



-Diagrammatic sagittal section through anterior end of Fowl embryo 

 with 15 segments. 



a.e, rudiment of amnion ; br, brain ; ect, ectoderm ; enc, endocardium ; end, endoderm of yolk-sac ; 

 f.g foregut ; h.f, posterior limit of head fold of ectoderm ; me, myocardium ; N, notochord ; pa, 

 ectoderm of proamnion ; splc, splanchnocoele. 



floor of the rhomboidal sinus the primitive streak is still visible 

 separated by a knob-like elevation from the part of the primitive 

 streak which lies farther back. 



The mottled appearance characteristic of the vascular area is 

 now seen to be continued inwards, though much more faint, across 

 the pellucid area to the body of the embryo. 



An embryo with about ten segments is shown in Fig. 231. 

 The pellucid area is still somewhat fiddle-shaped with the body 

 of the embryo lying along its axis. Apart from the increase 

 in number of the mesoderm segments the most conspicuous 

 advances in development are in the central nervous system. The 

 medullary folds have met and fused together to enclose the neural 

 tube except towards their hind ends where they still bound the 

 rhomboidal sinus on each side. The forebrain region is greatly dilated, 

 its projection on each side being the optic rudiment (o.r). It will 

 be noticed that a slight notch in its wall in the mesial plane anteriorly 

 indicates that at this point the two neural folds have even yet not 



