350 F. L. LANDACRE 



lowing the breaking down of the dorso-mesial portion of the lateral 

 mass present the appearance of not having broken down so com- 

 pletely as surrounding areas. A comparison of figs. 40 to 43, 

 (A. nebulosus, Stage VII) all from the same embryo, brings out 

 these relations in an embryo in which the ganglionic regions are 

 unusually well marked. Fig. 40 is taken one section posterior 

 the auditory vesicle. The whole region between the cord and the 

 endoderm is occupied by the derivatives of the lateral mass. Just 

 dorsal to the endoderm (fig. 40, Y) the tissue is somewhat dense 

 and there is also a slight condensation (fig. 40, G) at the level of 

 the upper third of the medulla; this condensation extends into the 

 next section (fig. 41 , G) but disappears in the section following that 

 from which fig. 41 was taken. From this point back to the region 

 where the Xth ganglion appears the lateral mass presents the 

 appearance shown in fig. 42. The mesectoderm presents the 

 appearance ofmesenchyme having well defined nuclei but with ill- 

 defined cytoplasmic branches forming a loose network. Just 

 over the mesoderm the derivatives of the lateral mass in all three 

 figures are somewhat denser. It will be recalled (pp. 330-331, 345) 

 that it is this portion of the lateral mass that forms the Gasserian 

 and geniculate ganglia in the preauditory region. 



Figure 43 is taken through the region of the Xth ganglion just 

 anterior to the anterior end of the lateralis Xth. This section 

 is taken just back of the anterior end of the ganglionic mass 

 (J. G. X, fig. 43) . The anterior end of the ganglion lies somewhat 

 nearer the medulla than in the section figured, while posterior to 

 this point the mass is found more ventral and lateral in position, 

 finally coming into contact with the denser portion indicated in 

 figs. 41 and 42, (F), lying just over the mesoderm. The loose 

 mass of cells shown in fig. 43 (J. G. X) gives rise to the jugular 

 ganglion to be described later. 



Like the early stages of the Gasserian ganglion, the whole mass 

 is extremely ill-defined with irregular borders passing gradually 

 into the surrounding mesectoderm. The ganglionic mass does 

 not reach the dorsal portion of the medulla and there is no indica- 

 tion of a migration of cells from that structure. 



