EXTENT OF THE FLOOR-PLATE OF HIS 127 



motorius similarly occupies the preaxial portion of the basal 

 plate with the nucleus trochlearis immediately caudad of it. 

 The olfactory areas (and cerebral areas) in the alar plate would 

 occupy positions morphologically caudad of the retinal area. 

 It by no means follows that these areas are more than potentially 

 present in the neural plate. It would be useless, of course to 

 attempt to assign even relative locations for other brain regions 

 without ascertaining the effects that the great growth, in length 

 and breadth, and also the unequal growth, would have. 



Certain comments may be made on the relations outlined by 

 the diagram and what they signify. 



1. The experiments of W. H. Lewis and Stockard have shown 

 that the retinal area (or areas) occupy the extreme cephalic por- 

 tion of the neural plate. There is not agreement as to whether 

 the area is primarily single or double, but this has little signifi- 

 cance in the present connection, although, if I may state it, my 

 individual opinion is that the evidence supports Stockard's con- 

 tention of a primarily single medial retinal area in the neural 

 plate. Johnston, from purely morphological studies, has indi- 

 cated that the cephalic boundary of the neural plate marks the 

 site of the future optic chiasma behind which as the retinal areas 

 separate the primitive optic furrow remains. 



2. The closer allocation of the motor and sensory sides of the 

 visual apparatus than would be otherwise possible is also quite 

 suggestive and renders somewhat more comprehensible a feature 

 of head morphology otherwise obscure. 3. The eyes thus would 

 represent the most anterior of the series of sense-organs. This is, 

 I think, no new interpretation, borne out as it is by the reversed 

 relation of eye to olfactory organ in Cyclopia. It also serves to 

 render more comprehensible the seriation of sense organs — retinal, 

 olfactory, otic, etc. — which Duval ('00) so nicely set forth in dia- 

 grammatic form in his figure 420. 



4. Possibly, though perhaps not probably, the existence of a 

 nucleus centralis n. oculomotorii, effecting a confluence of the 

 oculomotor nuclei of the two sides, may be thus accounted for, 

 as well as the medial position of other nuclei in this region (n. 

 centralis superior, n. centralis raphae, ganglion interpeduncu- 



