FUNDAMENTAL PLAN OF VERTEBRATE BRAIN 477 
intermediate to those of 9 and 10, 13 and 14, and 15 and 16, 
respectively. Figures 23 to 26 are of progressively older stages, 
all of them, however, older than that represented by figure 16. 
While in figure 17 the notochord is included in the entoderm 
and its prospective cephalic end is indeterminate without a 
comparison with the mesoderm and with later stages, in the 
succeeding stages figured it is separated off save at its cephalic 
and caudal ends where prechordal plate and marginal zone 
(blastoporic lip) are united to it. Both of these zones mark 
regions where notochord, paraxial mesoderm and entoderm are 
confluent. In the case of the marginal zone, of course, the 
neural plate must be added to these. The prechordal plate is a 
region of great significance in understanding the morphology of 
the head. Without stopping to consider the neural plate-noto- 
chordal plate relations more closely at this point, it is obvious 
at once from a survey of these figures that not only does the 
anterior end of the notochord fail to reach the anterior end of the 
neural plate, but falls far short of the primitive infundibular 
fold, being separated from it by the greater extent of the pre- 
chordal plate (ef. figs. 17 to 20) in which it ends. 
Turning to a more exact comparison of notochord and neural 
plate, in order to determine if possible the point in the neural 
plate with which the anterior end of the notochord primitively 
corresponds, it should be noted that only in the last figure of the 
series (fig. 26) is the fovea isthmi and the cephalic end of a dif- 
ferentiated floor plate distinguishable, where it has been desig- 
nated by the letter F. The interval of the medial stretch of 
floor between this point and the tuberculum posterius of von 
Kupffer is in the shark embryo of this stage extensive. P marks 
the latter landmark. No significant alterations in form develop- 
ment take place between this stage and the 40-mm. embryo 
figured in my earlier paper (figs. 1 and 8), with which it may be 
compared. Each preceding earlier stage has been compared 
and the equivalent point similarly designated, back to and in- 
cluding that of figure 19. In this comparison the descriptions 
and figures of Neal (’98), von Kupffer (’06), and Secammon (711) 
have been considered and compared, particularly the fine plates 
