FUNDAMENTAL PLAN OF VERTEBRATE BRAIN 483 
The bilateral growth characteristic of the mesoderm is clearly 
also a marked feature of the growth of the neural plate and is 
indicated in the comparison of figures 6 to 10 particularly. Also, 
it is precociously so in chick and mammals, and this is unques- 
tionably a factor in the difficulty with which the primitive rela- 
tions are recognized. 
The interpretation of the neural plate here presented furnishes 
also an adequate basis for the understanding of the morphology 
of the head as a whole. Certain features of cranial morphology 
that may be better comprehended on the basis of the present 
interpretation were indicated in my earlier paper, and since it is 
the intention to review the matter in a subsequent publication, 
it will not be further discussed here. As far as the brain tube 
is concerned, the outstanding feature, in addition. to the marked 
bilateral growth already referred to and the well-known foldings 
of the tube as a whole due to growth, is the great expansion of 
the dorsal portion, effecting a rotation, as it were, roughly around 
the region of preaxial mesoderm as a center or axis. This brings, 
thus, dorsal regions ventral, ventral aspects more dorsal, more 
caudal material cephalad—thus reversing the primitive sequence, 
with the more rostral expansion primitively dorsal. Certain of 
these rotations are more extreme in the embryo than in later 
stages. 
Little need be added to what was previously said upon the 
effect of the interpretation upon the relations of alar plate, basal 
plate, and sulcus limitans in the prechordal portion of the neural 
tube. The boundary between them as primary sensory and 
motor zones must necessarily be here indefinite and perhaps 
indeterminate. The boundary clearly lies, I think, in the region 
of the mammillary recess. The question, however, possesses no 
embryological bearing and I gladly leave it to the consideration 
of neurologic workers. 
Only as a comment at this time, it may be said that the mode 
of growth of prechordal neural plate and preaxial and paraxial 
mesoderm commented on above furnishes us with a suggestive 
basis for understanding the morphogenesis of the hypophysis. 
