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AN INTRODUCTION TO ZOOLOGY 



of the nerves. As the blastopore closes it becomes slit -like and 

 then fuses in the middle region, leaving an opening at each end. 

 The neural plate extends right back to the dorsal lip of the blastopore 

 and as it bends over and fuses to form the neural tube, it gradually 

 closes over the posterior end of the upper aperture, but internally 

 leaves at this point an open communication, the neurenteric canal, 

 leading from the neural canal to the enteron as in Amphioxus. 



Where the lips of the blastopore with their variou%approximated 



FIG. 134. Rana, young embryos. From Kellicott, after F. Ziegler. 



A, blastopore nearly closed ; neural folds just indicated. B, blastopore becoming divided into 

 neurenteric and proctodaeal portions ; neural folds becoming elevated. C, neurenteric canal 

 forming ; neural folds closing together. D, neural folds in contact throughout. 



b., blastopore, containing yolk plug ; bi, rudiment of neurenteric canal (dorsal part of blasto- 

 poreX ; bz, rudiment of proctod;val pit (ventral part of blastopore) ; ba, branchial arches ; g., 

 neural groove ; nf, neural folds ; np, neural plate ; p., proctodaeal pit ; x., neural folds roofing 

 the bkistopore and establishing the neurenteric canal. 



layers fuse, the cells of the ecto-, ento- and mesoderm are brought 

 together into one undifferentiated mass known as the primitive 

 streak down the middle of which runs a groove, the primitive groove 

 marking the point of junction. The lower opening fuses internally, 

 and so leaves an external pit lined with ectoderm. This is the 

 proctodceum, and internally an evagination of the entoderm, the 

 future rectum, grows out towards it. 



Practically while the neural folds are closing the brain region 



