CHAPTER EIGHT 



Neurulation and Early Or^aiio^eny 



Neurulation 

 Early Organogeny 



Surface Changes 



Visceral Arches 



Origin of the Proctodeum and Tail 



Internal Changes 



Neurulation 



The axes of the embryo are altered by the development of the 

 archenteron. The antero-posterior axis is made obvious by the de- 

 velopment of the neural axis. Either the roof of the archenteron 

 and/or the notochord induces, in the overlying ectoderm, the forma- 

 tion of a thickening, limited to the nervous layer. This becomes the 

 medullary (or neural) plate which extends from the dorsal lip of the 

 blastopore in an anterior direction as a median band of thickened 

 ectoderm which widens anteriorly where the brain will develop. 



Shortly the more or less parallel lateral margins of the thickened 

 medullary plate become even more thickened as the lateral folds 

 or ridges, and are continued anteriorly as the transverse neural fold 

 or ridge. A longitudinal neural groove or depression appears in the 

 center of the medullary plate, so that the height of the neural folds 

 appears to be accentuated. This is the very beginning of the forma- 

 tion of the central nervous system, induced by the presence beneath 

 of the archenteric roof and/or the notochord. 



The following description covers the period from the time the 

 medullary plate first thickens until about the time the embryo reaches 

 the 2.5 mm. stage. At this time the embryo shows ciliary movement 

 within its jelly capsule. These cilia are lost, except on the tail, by the 

 time the 1 1 mm. stage is reached. 



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