268 



C. W. PRENTISS 



an opaque object by reflected light to obtain the contour of the 

 different structures. By making several dissections of the same 

 stage I believe that the finer structures were more accurately and 

 completely reproduced than could be done by serial reconstruc- 

 tions. 



DESCRIPTION OF DISSECTIONS 



Stage 1. 6 mm. In this embryo (fig. 1) the ganglia were con- 

 nected from the glossopharyngeal (IX) back to the caudal region 



crist 



qanq 

 Froriep 



I*- gang sup. 

 , qangjugul 



it 



- gang, pet roj 

 I 



Fig. 1. Dissection of a 6 mm. pig showing in the hypoglossal region the undif- 

 ferentiated neural crest. See explanation of figures on page 282. 



by continuous bands or loops of cells, undifferentiated portions of 

 the neural crest. The ventral roots of the spinal nerves (d C 2 ) 

 are large, but the dorsal ganglia show little differentiation into 

 fibers, though short distal and proximal roots are present. The 

 hypoglossal originates as five or six parallel roots, resembling 

 those of the spinal series but uniting, in the later embryos of this 

 stage, to form a common trunk (XII). The spinal accessory 

 (XI), as an arched bundle of fibers, could be traced from the 

 fourth cervical ganglion cephalad to the vagus. Dorsal to the ac- 

 cessory and partly ensheathing it is a flattened band of cells, the 



