398 MABEL BISHOP 



wliich, it will be remembered, began as compressed tissue between 

 the prongs of the inverted- Y-shaped canal of the lower medulla, 

 and by rostral spreading forms at the level under consideration 

 the floor of the conjoined fourth ventricle. Both the sections 

 and the model disclose partial doubling in this area, as demon- 

 strated by a sulcus limitans medial to each raphe (figs. 5, 9, 10, 

 11). This ventricular floor consists, therefore, of two dorsal 

 and two ventral quadrants, of which the latter become increas- 

 ingly larger toward the isthmi and bulge into the ventricle, while 

 the dorsal quadrants become correspondingly smaller and insigni- 

 ficant and are seen to lie in the median sagittal plane of the 

 monster (figs. 9 to 15). More rostrally a cross-section through 

 the embryo at about the level of the cephalic flexure shows that 

 the dorsal quadrants have become so reduced that, only a mid- 

 dorsal choroid plexus remains, lying in the medial plane of the 

 ventral surface of the teratological rhombencephalon. Still 

 further forward this is seen to join the choroid plexus of the 

 dorsal (normal) surface, thus forming a midline choroidal arch 

 in the 'mound' between the mesencephala (figs. 5, 8). 



It is obvious, then, that the left half of head A, instead of 

 uniting with the right half of its own head, is joined to the right 

 moiety of head B along the middorsal surface of the conjoined 

 dorsal quadrants. This identification is important since it 

 establishes three teratomorphological points, namely, the median 

 sagittal plane of the monster, the line of fusion of the two heads, 

 and the line of bilateral symmetry. Other evidence in support 

 of the interpretation of the quadrants between the two raphes 

 will be added when the central connections of the cranial nerves 

 are discussed. 



The gross anatomy of the mesencephala and telencephala 

 may be dismissed with the brief statement that they give every 

 appearance of normality, although in any given microscopic 

 section there may seem to be asymmetry due, it will be remem- 

 bered, to the downward tilting of head A with consequent obli- 

 quity in the plane of section. 



