EARLY ORGANOGENY 



137 



Medullary plate 



Neural 

 groove 



Notochord 



Somite 

 mesoderm 



Proliferating 

 mesoderm 



Limit of 

 mesoderm 



Gut 

 endoderm 



Yolk 



Neural fold 



Neural crest 



Coelom 



Somatic 



mesoderm 



(epimere) 



^Splanchnic 

 mesoderm 



Lateral 



plate 

 mesoderm 

 (hypomere) 



2.0mm embryo 



2.3mm. embryo 



Neural crest 

 Neurocoel 

 Ner\/e cord 



Splanchnopleure 



Somatopleure 



Somite (segmental) 

 mesoderm 



Coelom 



Subnotochordol 

 rod 



Midgut 



Somatic mesoderm 



Splanchnic mesoderm 



Yolk 



2.6mm. embryo 



Formation of the neural groove and neural tube from the neural (medullary) 



plate. 



"nervous ectoderm" gives rise to tlie neuroblasts of the central nerv- 

 ous system. 



The roof and the floor of this neural tube are relatively thin, but 

 due to this nervous layer the lateral walls are very thick. The roof 

 is simply the region of the junction of the two neural folds. Some of 

 this nervous layer, at the level of dorsal fusion, is pinched off on either 



