334 



PROFESSORS ARTHUR ROBINSON AND A. GIBSON. 



the more cranially situated portion of the bulging pericardial region. Extending 

 caudally, from the dorsal border of the caudal part of the pericardial region, is the 

 ridge, already noticed from the dorsal aspect, which curves round the bend of the 

 embryo and indicates the position of the pleuro-pericardial canal of the corresponding 

 side (fig. 56, PL XIX). 



The Nervous System. 



The neural tube is closed and it is separated from the surface ectoderm except 

 at its caudal end. There it opens into the amniotic cavity through a narrow cleft- 

 like caudal neuropore 100/a long (figs. 54, 57, 59, PL XIX, and text-fig. 22, below). 



There is no sharp line of separation between the cerebral and spinal portions of 



Left dorsal 

 Aorta 



Ca udaJ neuwpow 



Allantoic mesoderm^ 



Somatopleurv 



Allantoic mesoderm 



Allantoic entoderm 



Text-fig. 22. — Dorsal view of a dissection of the caudal end of the model of the embryo, showing the caudal 

 neuropore, the terminal part of the left dorsal aorta, and the allantoic mesoderm and entoderm. 



the tube, but the cerebral portion is clearly demarcated into hind-, mid-, and fore-brain 

 sections, of which the hind-brain is the largest and the mid-brain the smallest. On 

 account of the mid-brain flexure the fore-brain forms an obtuse angle with the hind- 

 brain, and it lies in the anterior boundary of the fore-gut and the stomatodseum 

 (fig. 57, PL XIX). It is separated from the mid-brain by a slight internal ridge and 

 a less marked external sulcus. It shows no signs of segmentation ; but from each 

 side of its cranial end a primary optic vesicle bulges laterally, forming a projection 

 which is visible from the external surface (fig. 36, PL XV). 



The mid-brain also shows no signs of segmentation. It is a tube of fairly uniform 

 calibre, and it passes without any sharp line of separation into the hind-brain. 



The hind-brain is curved on its long axis, the concavity of the curve being 

 directed dorsally, and, as already stated, no definite line can be fixed at which it 

 becomes continuous with the spinal medulla. Still, in the region immediately caudal 



