156 



THE DEVELOPMENT OF THE CHICK 



ment of the nuclei leaves thin non-nucleated strips (septa) be- 

 tween adjacent neuromeres. The interneuromeric projections are 

 most pronounced laterally and fade out dorsally and ventrally. 



Behind the neuromeric portion of the hind-brain is a portion 

 extending to the posterior end of the fourth mesoblastic somite 

 from which the twelfth cranial nerve arises. 



The Neural Crest and the Cranial and Spinal Ganglia. The 

 cranial and spinal ganglia owe their origin to a structure known 

 as the neural crest, which is a practically continuous cord of cells, 

 h^ng on each side in the angle between the neural tube and 

 the ectoderm, extending from the extreme anterior to the pos- 

 terior end. Like other meristic structures the anterior portion 



Fig. 89. — Frontal section of the hind-brain region of an embryo of about 

 36 s. 

 Ot., Otocyst. N. 6, N. 7, N. 8, N. 9, N. 10, N. 11, Neuromeres, 6 to 11, 

 according to Hill's enumeration, s. 1, s. 2, s. 3, First, Second, and third 

 somites. V, Primordium of the trigeminus. VII-VIII, Primordium of the 

 acustico-facialis. 



of the neural crest is the first to arise (at about 6-7 s stage), 

 and the remainder appears in successive order during or shortly 

 after the closure of the neural tube in each region; thus it is not 

 until after the completion of the neural tube that the last portion 

 of the neural crest is established. 



But before this time successive enlargements of the cranial 

 part of the crest have formed the primordia of the cerebral gan- 

 glia, and similar successively arising enlargements of the parts 

 of the crest opposite the mesoblastic somites form the rudiments 

 of the spinal ganglia. The intervening portions of the crest form 

 the so-called interganglionic commissures, which subsequently 



