i8o 



EMBRYOLOGICAL TYPES 



lips, and its former aperture is represented only by a short 

 groove on the outer surface between these lips. This fact is 

 of importance in connexion with the interpretation of develop- 

 ment in higher forms. The activity of the cells of the 

 blastopore-rim continues after the blastopore is closed, and 

 leads to the outgrowth of the tail. 



Nerve-tube. — The ectoderm along the middle line of the 

 dorsal side thickens forming the neural plate. A pair of 

 longitudinal ridges arise on each side of it, known as the neural 



Fig. 80. — Sagittal section through an embryo of Rana. 



a, anus ; b, brain ; g, gut ; h, hypophysis ; ht, heart ; /, liver 

 notochord ; sc> nerve- (spinal) cord ; y, yolk-cells. 



folds, and they enclose a groove between them. This groove 

 is wider in front, where the brain will be, than behind, in the 

 region of the future spinal cord. Posteriorly the neural folds 

 embrace the blastopore. As the folds rise up they arch over 

 the groove, which becomes converted into the nerve-tube. 

 The nerve-tube communicates with the cavity of the gut 

 posteriorly through the neurenteric canal and blastopore, which 

 is still open at this stage. 



The lateral part of the thickening which gave rise to the 



