CHAPTER NINE 



A Survey or tire Major Developinental 

 CJran^es in tlie Early Embryo 



Primary Divisions of the Brain Origin of the Somites 



The Enteron or Gut Cavity Origin of the Excretory System 



The Axial Skeleton Origin of the Mesodermal Epithelium, 

 The Mesoderm and Its Derivatives Coelom, and Its Derivatives 



Origin of the Arches Origin of the Heart 



Primary Divisions of the Brain. 



The anterior vesicular expansion of the central nervous system 

 becomes constricted at certain levels and the walls begin to differen- 

 tiate and may be used as identifying landmarks of the embryonic 

 brain. It has just been stated that the brain floor bends ventrally 

 (ventral or cranial flexure) around the tip end of the notochord. This 

 flexure remains as a permanent feature of the brain. The brain floor 

 at this region is known as the tuberculum posterius (posterior tu- 

 bercle), and is in line with the notochord and the anterior neuropore, 

 and will give rise to the floor of the mesencephalon or midbrain. 

 Slightly anterior and dorsal to the tuberculum posterius the roof of 

 the brain acquires a considerable thickening for a limited distance. 

 This is known as the dorsal thickening and will be identified as the 

 roof of the mesencephalon, later to give rise to the optic lobes. The 

 primary brain very rapidly develops thinnings and thickenings of its 

 walls, invaginations, and evaginations, all of which are part of its 

 differentiation. 



It is now possible to de-limit the three primary parts of the embry- 

 onic brain. These divisions are present in all vertebrate embryos 

 of a comparable stage of development. They are the prosencephalon, 

 mesencephalon, and rhombencephalon. 



Prosencephalon. This is the primary forebrain, consisting of all 

 parts of the brain anterior to a line drawn from the tuberculum pos- 

 terius to the anterior limit of the dorsal thickening, largely anterior 

 and ventral to the notochord. This portion of the brain develops al- 



139 



