482 B. F. KINGSBURY 
The notochord terminates anteriorly in a zone of proliferation 
where mesoderm and entoderm are for some time confluent. 
The material in which the anterior end of the notochord termi- 
nates, the prechordal plate, is actually or potentially preaxial 
mesoderm and furnishes the material from which differentiate 
the muscles innervated by the oculomotor (IIIrd cranial) nerve. 
The paper of Adelmann, several times referred to, critically 
examines the growth transformations in this region and furnishes 
us with an adequate conception of the significance of the pre- 
chordal plate. While the prechordal plate is primitively pre- 
axial, its growth is largely bilateral rather than in the median 
plane, a feature important in understanding the morphology of 
the head. Thus, if the series of medial planes in Squalus (figs. 
17 to 26) are compared, it is seen that the preaxial mesoderm 
which separates off from the entoderm and gains a cavity (con- 
necting the cavities of the premandibular somites), becomes 
eventually drawn out to the sides and disappears without con- 
tributing directly to preaxial median structures, unless indeed 
the most anterior portion connecting the ‘anterior’ head cavities 
may persist (cf. Adelmann, ’22). 
The bilateral growth appears more precocious in the higher 
vertebrates, particularly mammals, disguising the significance 
of primitive relations and rendering an appreciation of the mor- 
phogenetic pattern difficult. With the disappearance of medial 
mesoderm the notochord may through growth attain secondary 
relations quite different, as fusion with the hypophysis (cf. 
Rand, ’17, et al.). In the chick, likewise, the attenuation of 
the medial preaxial mesoderm may give rise to faulty interpre- 
tations if one were to omit comparison with birds, such as the 
duck, in which the differentiation is more marked (cf. Adelmann). 
The slight development of the medial preaxial mesoderm in the 
chick as compared with the shark may be seen on comparison 
of the two sets of median planes, plates 2 and 3. It should be 
appreciated, I think, that we are dealing with a mode of dif- 
ferential growth possessing marked plasticity, hence interpreta- 
. tion in terms of a rigid morphology is not adequate. 
