422 MABEL BISHOP 



ganglia are suggested, but with an uncertainty that warrants 

 caution. 



9. A pair of cerebral hemispheres, a diencephalon, and mesen- 

 cephalon are normal for each head member. They diverge 

 rostrally from the rhombencephalon (figs. 4, 6) . 



10. The rhombencephalon is conjoined, larger than in a normal 

 embryo, but in proportion to the size of the teratological embryo 

 (fig. 6). _ 



11. The principal abnormalities of the head region are asso- 

 ciated with the rhombencephalic region. 



12. The conjoined metencephala form a 'mound' between the 

 mesencephala (figs. 6, 8). 



13. The central canal of the lower medulla, as seen in the 

 sections, has the shape of an inverted Y (figs. 13, 14). 



14. The fourth ventricles of the two head members are con- 

 fluent (i.e., conjoined) and therefore larger than normal (figs. 

 9, 10). There is a lateral recess at each outer side, but none in the 

 conjoined region (figs. 9, 10). An embryonic cerebellum bulges 

 into the ventricle from the dorsal quadrants of the outer walls 

 (figs. 5, 10), but no cerebellar swellings are apparent in the 

 median region. The conjoined dorsal and ventral quadrants of 

 the left and right halves of heads A and B, respectively, form a 

 floor to the conjoined ventricles (figs. 9, 10). The quadrants 

 lie side by side and are separated by a sulcus limitans, as are the 

 dorsal and ventral quadrants of the normal (outer) sides (figs. 

 9, 10, 11). In the teras the ventral quadrants of the median 

 region become increasingly larger toward the isthmi and bulge 

 into the ventricle, and the dorsal quadrants correspondingly 

 smaller and squeezed into the median line of the monster so that 

 far rostrally the middorsal choroid plexuses of heads A and B 

 lie dorsal and ventral to each other (fig. 8) and by their fusion 

 form a narrow choroidal arch in the median sagittal plane of the 

 metencephalic mound (fig. 5). A thin membrane forms the 

 dorsal boundary of the broader portion of the ventricle, but on 

 either side there is a well-developed choroid plexus extending 

 into each lateral recess (figs. 9, 10). 



