264 The Nature of Biological Diversity 



of this column and the segregation of its different components involve 

 a series of developmental steps which have been analyzed in detail 

 (4) and will be summarized here. In this initial phase of their matura- 

 tion, the neuroblasts move as isolated units. They cover the short 

 distance between the central canal and the lateral edge of the neural 

 tube by means of ameboidic movements of their bodies, which glide 

 between the parallel and tightly packed rows of undifferentiated cells. 

 As soon as they reach the ventrolateral area of the tube, they assemble 

 together in a column which increases in size as new units join the 

 ones which have already settled. This process continues till the fourth 

 day. A similar migratory movement takes place also from other more 

 dorsal sectors of the ependyma around the central canal. The neuro- 

 blasts originating from these segments do not gather in a column but 

 take position as isolated units or small cell groupments along the 

 dorsolateral edge of the neural tube. They give origin to the associa- 

 tive and commissural neurons, which establish links between the two 

 halves of the spinal cord and between rostral and caudal segments of 

 the tube. At the end of the fourth day of incubation, this early differ- 

 entiative phase is practically completed in the brain centers and in 

 the rostral segments of the spinal cord, while it is still in progress but 

 near completion in the thoracic and lumbosacral segments which 

 mature later, according to the rostrocaudal gradient of differentiation. 

 The subsequent phase, between the end of the fourth and the eleventh 

 day of incubation, represents the most dynamic period in the devel- 

 opmental history of the spinal cord and brain centers of the chick 

 embryo. It is in this period that the apparent homogeneous popula- 

 tion of early differentiated nerve cells breaks down at some levels and 

 is swept away in a few hours, while at other levels it splits in mor- 

 phologically and functionally distinct cell agglomerates; some of the 

 cells continue their differentiation in situ while others migrate in 

 compact columns or cell rows to settle in regions at considerable dis- 

 tance from the ones where they first underwent maturation. Numer- 

 ous instances of such migratory processes were described in different 

 segments of the spinal cord, brain stem, and higher brain centers 

 (4, 5, 6) . Here we shall outline only the main developmental patterns 

 of differentiation of nerve centers and present some considerations 

 on the mechanism of these differentiative processes. 



b. Second differentiative phase 



At the end of the fourth day of incubation, the spinal cord in the 

 chick embryo exhibits a deceptively simple architecture. A compact 



