268 The Nature of Biological Diversity 



distinct nuclei adjacent to the central canal the cells of origin of the 

 preganglionic motor centers (Figs. 1, 10, and 11). 



An Experimental Analysis of the Regional 

 Patterns in the Cervical and Thoracic 

 Spinal Cord Segments 



The hypothesis (see page 265) that the disintegrating nerve cells 

 in the cervical segment of the spinal cord might represent an ahortive 

 visceral column was tested by transplanting the cervical segment of 

 the spinal cord from 2-day chick embryo to the thoracic level of hosts 

 of the same age (8). The thoracic segment of the host was extirpated 

 and the transplant inserted in its place. In performing this operation, 

 Shieh wanted to test the hypothesis mentioned above, by providing 

 the "abortive cells" with the possibility of establishing connections 

 with the sympathetic ganglia. The operation presents technical diffi- 

 culties and only a few embryos survived till the seventh day. In the 

 few specimens examined, Shieh observed the formation of a pregan- 

 glionic motor column similar to the one which forms at the thoracic 

 level. He also observed a large number of degenerating neurons. These 

 results suggest that at least a number of the cells that normally un- 

 dergo degeneration at the cervical level survive in the transplant and 

 build a preganglionic motor column with the same mechanism as nor- 

 mally occurs in the thoracic level. These experiments, if confirmed 

 by others now in progress, will support the hypothesis that an abor- 

 tive preganglionic center is present in the cervical level of the spinal 

 cord. They would also give evidence of the flexibility of differentiative 

 patterns in the central nervous system. 



Mass Migration in Other Sectors 

 of the Central Nervous System 



Between the fifth and the eleventh day, the brain stem and the 

 suprasegmental brain structures are the stage of an incessant and yet 

 highly organized movement of differentiated nerve cell populations. 

 We shall consider below the main features of these movements, as 

 they appear in the developing nervous system of the chick embryo. 



Migratory Movements in the Brain Stem. The early continuous 

 motor column (see page 265) segregates at first in the primordia of 

 the XII, XI, X, VII, V, IV, and III nuclei. Each of these centers seg- 

 regates in turn in the following days into two or more cell groupments 

 through more or less complex migratory movements of its components. 



At some levels, movements in opposite directions take place at the 

 same time, or two cell populations, moving along slightly divergent 



