THE DEVELOPMENT Of MAMMALS 



641 



destined to become the hindgut, so the allantois of mammals has the 

 same relationship to the gut that the allantois has in reptiles and birds. 



261. Formation of the Notochord and Neural Tube 



All of the extraembryonic membranes characteristic of amniotes 

 (amnion, chorion, allantois and yolk sac) are now present, and the em- 

 bryo itself is beginning to take shape. A notochord develops beneath 

 the surface ectoderm in the longitudinal axis as the primitive streak 

 shortens and retreats toward the posterior end of the embryonic disc. 

 The ectoderm overlying the notochord thickens and becomes a neural 

 plate. The lateral edges of the neural plate are elevated as a pair of 

 neural folds, which gradually come together (Fig. 31.4). The inner 

 limbs of the folds become the neural tube, which differentiates into the 

 spinal cord and brain as described in section 247; the outer limbs, along 

 with the rest of the surface ectoderm, become the epidermis of the skin. 

 Ectodermal cells that are pinched off near the apex of each neural fold 

 form a ridge, the neural crest, on each side of the neural tube. The cells 

 of the neural crest become segmentally arranged and many of them 

 differentiate into the afferent neurons of the spinal and cranial nerves. 

 Other neural crest cells migrate and form postganglionic sympathetic 

 fibers (other types of efferent neurons grow out from the neural tube), 

 the medullary cells of the adrenal gland, the neurilemmal sheath cells 

 of peripheral neurons, and certain other structures. Surface ectoderm that 



NeuraJ— 1 

 pla.te 



S ui^f a.ce — 1 

 ectoderna 



Neurad fold 



Notochord 



Neural groove— 



B 



Neural 

 cre-st 



Figure 31.4. A series of cross sectional diagrams through the surface ectoderm to 

 show the formation of the neural tube and neural crest. (After Arey.) 



