198 
DR. MARTIN BARRY ON THE CHORDA DORSALIS. 
shows a “growing from” the sheath, as it is called, of the chorda, — “at the expense 
of its inclosed nucleus,” — or as the chorda is “found to decline,” even in these the 
last stages of this structure. But if we examine it at periods anterior to these, I 
think we shall find still stronger grounds for believing, that the chorda dorsalis of 
authors, in an early state, corresponds to the pin-like object in the mammiferous 
ovum which I have been comparing with it. 
Reichert seems to have been led to form the opinion above-mentioned, that the 
chorda serves as a “support and stay” for parts developed in two halves, by the fol- 
lowing observations ; namely, that the chorda becomes visible as a single structure at 
the same time as the foundations, in two halves, of the central nervous system ; and 
that the central nervous system on the one hand, and his inembrana intermedia on 
the other, are so intimately connected by means of the chorda, that it is not possible 
to separate them. He even states, that the chorda passes into the foundation of the 
embryo. 
Farther, Reichert says, “There are developed on the chorda dorsalis the original 
halves of the central nervous system, with the higher organs of sense, separating as 
these do from the central nervous system ■f- He states that, with a union of the 
two halves of the central nervous system — such union taking place first at the fore- 
end — there is observed a decline in the corresponding part of the chorda. On the 
subject of the membrana intermedia, the same author remarks, that it is found be- 
tween the central nervous system and the mucous membrane ; and that it (the mem- 
brana intermedia) “is the common original foundation of all structures, systems, 
and organs, which are the means of operation for the two central organs of animal 
life. Hence from it [the membrana intermedia] there are developed the vertebral 
system, the dermal system, the circulating system, and finally, all the structures 
which support the mucous membrane, and which,” says he, “I comprehend under the 
name of system of the intestinal membrane;};.” 
It would thus seem, according to Reichert, that there is little in the embryo which 
is not developed out of either the central nervous system, or the membrana inter- 
media. But these are the very parts which the same observer found so intimately 
connected by means of the chorda, that it was not possible to separate them. And it 
appears to be these same parts called by Reichert “ the foundation of the embryo,” 
into which he says “ the chorda passes .” 
Taking then the observations of Baer, Rathke, and Reichert, in connection with 
my own, I venture to believe, that it is not enough to say, with Baer, that the chorda 
dorsalis is the axis around which the first parts of the foetus form ; nor, with Rei- 
t L. c., pp. 58, 59. 
\ L. c., p. 107. “Da die ferneren Doppel-gebilde,” says Reichert, “ sammtlich von der membrana inter- 
media ausgehen, so tritt die Wirbelsaite [chorda dorsalis] in die innigste Beziehung zu der Letzteren.” 
