104 BULLETIN OF THE 
head-mesoderm, and thus, through the obliteration of the first two 
protovertebre, the original third would now be recognized as the 
most anterior. The mass of the obliterated protovertebra, which, 
however, has not been decreased in volume, but whose contour as proto- 
vertebrw has disappeared, is now recognized as head-mesoderm, and 
has thus augmented the space intervening between the auditory vesicle 
and the now most anterior (original third) protovertebra. It is highly 
probable that the last two processes enumerated, namely, the prolifer- 
ation of head-mesoderm and the degeneration of protovertebrs;, com- 
bine to produce the apparent removal of the first protovertebra from 
the auditory vesicle, or the increase in the space intervening between 
these structures, It is certain, at any rate, that there is at this time 
an active cell proliferation in the head-mesoderm of this region, and that 
the first and second protovertebre become less distinct and more or less 
imperfect subsequent to their early formation. 
(^) Topography of the Embryo. — For convenience in description and 
reference to the different regions of the embryo, I have adopted the fol- 
lowing general divisions in the topography of later stages : — 
(1) The head region includes all of the embryo from its anterior end 
to the posterior limit of the auditory vesicle. 
(2) The pectoral region includes all from the posterior limit of the 
auditory vesicle to the nephrostome. 
(3) The abdominal region embraces all of the embryo included be- 
tween the nephrostome and the point of the future anus. 
(4) The caudal region includes all back of the anus. 
This classification is slightly different from that adopted by Ziegler 
(87, p. 643). The boundary between the head region and pectoral 
region cannot be definitely fixed, inasmuch as the position of the first 
protovertebra is changeable. Although not embraced in the region of 
the head by this classification, the axial mesoderm between the auditory 
vesicle and the first protovertebra is designated as “ head-mesoderm.” 
As will be seen iater, this region is probably involved in the origin of 
the pectoral fin. 
(c) Differentiation and early Development of the Mesoderm in the Peeto- 
ral Legion. — The changes which take place in the pectoral region during 
the earlier embryonic stages are of special importance in tracing the 
exact topography of the mesoderm, and the necessity of a reliable start- 
ing point in the topography will be apparent in subsequent consid- 
erations. The differentiation of the mesoderm into protovertebrie 
can be readily traced from the third or fourth protovertebra into the 
