NERVOUS SYSTEM OF A TWO-HEADED PIG EMBRYO 



397 



no indication of them in the conjoined region (fig. 10). The 

 tegmen is of normal thinness. The choroid plexuses of the two 

 heads are conspicuously developed, united in the middorsal 

 line, and extend into the lateral recesses in normal manner 

 (figs. 5, 9, 10). Any cross-section through the conjoined fourth 

 ventricle will show distinctly a median raphe for each member, 

 which marks the median sagittal plane of heads A and B, respec- 



Fig. 11 Section through the superficial origin of the trigeminal nerves of the 

 outer (normal) sides of the embryo, and the median optic nerves. S. 204. 



tively, that is, the normal morphological axis of each head. At 

 the outer side of each raphe is seen a distinct sulcus limitans 

 dividing the walls of the rhombencephalon into dorsal and ventral 

 quadrants (figs. 5, 9, 10) and in some of the sections the embryonic 

 cerebellum may be seen bulging into the ventricular cavity from 

 the dorsal quadrant of each normal side (figs. 5, 10). No cerebel- 

 lar swelling is identifiable in the conjoined or median region. 



It becomes necessary at this point to speak more fully of the 

 median area of the brain (i.e., the area between the two raphes) 



