NOTES ON BATRACHIANS OF THE PAEAGDAYAN OHACO. 323 



into two portions, the upper being the opening of the neuren- 

 teric canal (iV. en.), the lower the rudiment of the rectum 

 (Rect). 



Section IX shows the fusion of the layers in the region of 

 the neurenteric canal, and the separation of the latter from the 

 rectum. 



Section X of the same series, through the tail and vent, 

 shows the opening of the neurenteric canal into the neural 

 tube ; also the fusion of the epidermis with the hypoblast at 

 the anus {An.). 



Section XI, a transverse vertical section quite at the an- 

 terior end of the head of an embryo, in which the first pair of 

 external gills are beginning to bud. The section passes 

 through the bottom of the stomodseum {Stom.), and obliquely 

 through the mandibular arches at the point where they meet 

 {Mnd.). 



Section XII is of the same series, but further back, and on 

 the right side in the figure passes through the centre of one of 

 the eyes, showing the attenuation of the posterior wall of the 

 optic cup [Op. w.p.) and the thickening of the anterior wall 

 [Op. w. an.), the rudiment of the retina. The lens is also 

 seen arising as a regular involution of the nervous layer of the 

 epiblast, the epidermal layer (/.) remaining stretched across as 

 a very thin membrane. The section also passes through the 

 middle portion of the mandibular arches (Mnd.). The 

 pharynx and pericardium are also cut through {Ph., P.c). 



Section XIII of the same series passes through the centre 

 of the opposite eye. The proximal parts of the mandibular 

 arch are here cut through {Mnd.). The formation of the peri- 

 cardium and heart, with its mesodermal membranous lining, 

 is well seen (P.c, ht.). 



Section XIV of the same series passes a very little further 

 back through the infundibulum, pharynx, and the two lateral 

 extensions of the pericardium overlying the sinus venosus 



{S.V.). 



Section XV is of a slightly older embryo, passing trans- 

 versely through the eyes. The lens is now nearly completely 



