228 



ESSENTIALS OF ZOOLOGY 



plate is converted into the neural canal. The shutting in of 

 the neural canal and the neurenteric canal is followed by a 

 closing of the lower part of the blastopore, but this lower part 

 of the blastopore persists as a pit which marks the later forma- 

 tion of the anal opening. The blastopore is thus converted 

 above into the neurenteric canal and below into the anus. 



Neural fold 



Neurenteric canal 

 Proctodeum 



FIG. 116. External views of embryos to show the early condition 

 of the neural plate and the fate of the blastopore. 



After the closing of the neurenteric canal the neural canal 

 is extended as a tube. The anterior end of the tube is wide, 

 is bent downwards, and already presents the features of the 

 forebrain. The midbrain occupies the angle of bending, and 



the hindbrain is hori- 



Brain Notoch. Ent. Neurent. canal zontal and passes into 



the spinal cord. 



The endoderm, 

 after the formation of 

 the neurenteric canal 

 and the closing of 

 the blastopore, en- 

 closes the enteron as a 

 cavity which is closed 

 but communicates with 



the neural canal posteriorly. It consists of a single layer dors- 

 ally and laterally, but below its floor and sides are occupied by 

 the many-layered yolk cells. The growth which proceeds at 

 the posterior end forms the tail and extends the cavity beyond 

 the point which marks the position of the anus, and this 

 extension is called the postanal enteron and endoderm. The 



Mesenchyme Proctodeum 



FIG. 117. Diagrammatic longitudinal section 

 of embryo. After Marshall. 



