880 The American Naturalist. [October, 
and cuts, ante p. 618). It may be considered as still uncertain 
whether the sheet of mesoderm receives accretions at its distal 
edge from the yolk cells (entodermic) upon which it rests. There 
usually is no sharp limit between the two, and therefore we must 
consider it probable that at first the mesoderm receives additions 
from the yolk; later on it is found divided from the vitelline cells, 
and after it has split off it probably grows independently. The 
growth of the mesoderm at first from the yolk has been found in 
Petromyzon by A. E. Shipley, 38, 177-178 (of Studies), although 
in later stages the mesoderm is severed from the yolk. 
In later stages the mesoderm is wanting in the median line, 
and thus constitutes two masses or two lateral sheets. This bi- 
lateral division is effected by the development of the medullary 
groove and notochord. The mesodermic connection with the 
entoderm is retained, but is double, owing to the division. Along 
the median dorsal line of the archenteron runs the strip of ento- 
derm which forms the notochord ; on each side of this strip runs 
the line of connection between entoderm and mesoderm. The 
study of this secondary condition has led many authors into the 
error of ascribing a double 
origin to the amphibian meso- 
derm, and inferentially to the 
vertebrate mesoderm in gen- 
eral. This brings us to the 
P d 
rdi 
Tn, 
SER 
ie W'Z. 
puri. „> 
S 
= 
g | consideration of O. Hertwig’s 
Ee A E . 
BRO yee views, which form one of the 
ERSTE 3 chief supports of the “ Coelom- 
I AR teen] 
RL LTE | theorie” of the brothers Hert- 
NOUS EO SL EL 2) wig. For further discussion 
bc: Oe IR ic of this theory, see p. 893. 
DIR O. Hertwig, 72, 75, studied 
: FIG. 23.—Axolote embryo: transverse sec- stages in which the notochord 
pata ee = ee M ad dic had appeared, and at this 
groove; Ch, notochord; Enz, entoderm; Ach, time, as O. Schultze, 35, has 
— IE: Agere shown, the primitive relations 
of the layers no longer exist, but Hertwig regarded the second- 
ary arrangements in question as primary. He found no meso- 
