DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO 



Neurocoel Hindgut Midgut Notochord 



Foregut 



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Rectum 

 Proctodeum 



Optic vesicle 



Head mesenchyme 

 Oral evagination 



Heart mesenchyme 

 Liver diverticulum 



Three-dimensional representation of the late neurula stage of the frog, Rana 

 pipiens. (Redrawn and modified after Huettner.) 



The large cavity of the foregut is the pharynx, which expands 

 laterally to form endodermally lined visceral pouches on either side 

 of the developing vertical arches. These pouches are therefore verti- 

 cally elongated endodermally lined sacs, at all times continuous with 

 the pharynx. The most anterior pouch is called the hyomandibular 

 because it comes to lie between the mandibular and the hyoid arches. 

 Following this will be the first, second, and third branchial (gill) 

 pouches, otherwise known as the second, third, and fourth visceral 

 pouches. These all expand outwardly toward the opposed invagina- 

 tions of the ectoderm which form external visceral grooves. 



A medio-ventral and posteriorly directed pocket develops from 

 the foregut, extending beneath the yolk a short distance. This is 

 the liver diverticulum, the forerunner of the bile duct, the gallbladder, 

 and the liver. This is the extent of gut development by the 2.5 mm. 

 stage. 



The Axial Skeleton. 



The notochord was derived from cells indistinguishable from the 

 mesoderm at the region of the dorsal lip. These cells very quickly 

 expand and become vacuolated, and take on the appearance which 

 they will manifest throughout development until they are displaced 

 by bone of the vertebral column. Even the intercellular material of 

 the notochord becomes vacuolated. The entire group of cells be- 

 comes enclosed in an outer elastic sheath and an inner fibrous sheath, 

 both of which are classified as connective tissue. 



