476 B. F. KINGSBURY 
tures should be when first laid down coextensive. Since I have 
shown that a differentiated floor plate extends no farther for- 
ward than the fovea isthmi, the primary extent of the notochord 
would thus fall far short of the point marked out as its anterior 
end by His, namely, the point where sutura terminalis, sutura 
neurochordalis, ectoderm, and entoderm (i.e., pharyngeal mem- 
brane) adjoin (cf. the His fig. 1, 1892, 2 paper, reproduced in 
my earlier paper as figure 4). 
The question as to whether or not it is permissible to recognize 
a@ primitive suture, actual or potential, uniting the right and left 
halves in the epichordal portion of the body is a purely embryo- 
logical one and it is not proposed to enter into its discussion here. 
More pertinent at this time is the determination as to whether 
or not floor plate and notochordal plate are primarily coextensive, 
and hence whether, granting the propriety of the term suture, 
it may likewise be designated as neurochordal suture. This 
determination is somewhat difficult for two main reasons: 1) 
At the early developmental stage in which notochord and neural 
plate are in juxtaposition the floor plate is still undifferentiated. 
Likewise in most forms at least the anterior end of the notochord 
is not at first clearly demarcated from the prechordal plate 
(preaxial mesoderm). 2) The neural plate (tube) and notochord 
grow markedly and at different rates, the former obviously more 
rapidly, so that the neural tube becomes bent away and separated 
from the notochord so that by the time the fovea isthmi is evident 
and the floor plate distinguishable through its differential change, 
the points for comparison are quite remote from each other. 
These difficulties do not, however, render valueless a comparison 
of neural plate and notochordal plate and recourse may again 
be had to the median plane relations in progressively older 
stages. 
Shark. Sixteen median plane reconstructions were made of 
shark embryos from 1.5 to 23 mm. length, of which ten are re- 
produced as plate 2. The magnification is the same for all so 
as to permit more readily visualization of the growth changes. 
Figures 17, 19, and 20 are from the embryos shown in figures 6, 
11, and 13, respectively. Figures 18, 21, and 22 are of stages 
