DR. MARTIN BARRY ON THE CHORDA DORSALIS. 
197 
the so-called “ primitive trace,” by the formation of fresh substance in the interior 
(compare with my fig. 122.). If so, the formation of Baer’s incipient chorda is not 
simultaneous with, but subsequent to, that of his “ laminse dorsales.” (The “ laminae 
dorsales” of Baer correspond to Reichert’s “ central nervous system.”) 
Von Baer describes the chorda as “ the axis, around which the first parts of the 
foetus form-p-” The dark pin-like object we have been considering in the inammi- 
ferous ovum, presents merely one of the many layers of incipient cells into which a 
nucleus becomes resolved. 
Notwithstanding these differences, however, facts mentioned by authors regarding 
the chorda dorsalis at later periods I think afford evidence of the identity of the two 
objects in question, as will be seen by what follows. 
Rathke states that in osseous fishes the chorda dorsalis is inclosed in a membra- 
nous sheath ; and that from this sheath there grow in pairs a great number of minute 
filaments, which are the incipient crura of the arches of the vertebrae. The founda- 
tions of the bodies of the vertebrae appear, from the description given by this author, 
to consist of minute tables or traces, which proceed out of the vertebral arches ; and 
he adds, that “ the thickening and ossification of the vertebrae take place at the ex- 
pense of the inclosed nucleus of the chorda^.” 
Reichert, in his researches on the development of the Batrachian Reptile and the 
Bird, found the chorda to decline more and more as the vertebral system advanced 
in its formation §. ‘'The chorda,” says this author, “is reduced in proportion as 
ossification proceeds, until for the most part only its remains are to be found between 
the vertebrae ||.” 
It is known that the chorda dorsalis in a comparatively advanced state is composed 
of cells. For this discovery we are indebted to Professor J. Muller. Schwann has 
since found the characteristic nucleus in its cells “If we closely examine,” says 
this observer, “ the outer rind of the chorda in Pelobates fuscus — covered as this rind 
is with scattered grains — we find these grains closely to resemble the nuclei of cells; 
only that they are about half the size. They are besides oval, and furnished with a 
nucleolus. This rind is not distinctly separated from the proper tissue of the chorda 
dorsalis : and as the cells of the latter rapidly diminish in size towards the rind, I 
believe these grains of the rind to be the cytoblasts of flattened down cells, which 
form the rind-f-f”.” 
I think there is a great deal in what has now been quoted from these authors, that 
t L. c., p. 15. 
+ Burdach’s Physiologie als Erfahrungswissenschaft, 1837, Band II. pp. 279-281. 
§ L. c., p. 68. || L. c., p. 71. 
Mikroskopische Untersuchungen iiber die Uebereinstimmungen in der Struktur und dem Wachstkum der 
Thiere und Pflanzen, 1838, 1839, Tab. I. fig. 4. 
ft L. c., p. 12. It is deserving of notice that Schwann conjectures the chorda dorsalis to contain no ves- 
sels. 
2 D 2 
