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meridional plane of the fertilised egg. This point will be dealt 
with later. 
By sketching the marked eggs from time to time with the drawing- 
prism I obtained several series, which may e.g. give exact information 
on the appearance and displacement of the blastoporie rim. On this 
point the most divergent opinions until the present day are met with 
among different investigators. 
The oldest view is that the black hemisphere becomes the dorsal 
part of the embryo, so that the egg axis lies dorso-ventrally. As is 
well known Prrücer (1883) first pointed out that the blastupore 
moves forward over more than 90° from the point where the dorsal 
lip first appears, from which Prrücer concluded that the foundation 
of the nervous system originates on the white hemisphere. He 
added however emphatically: “Um nicht missverstanden zu werden, 
möchte ich hervorheben, wie ich keineswegs bewiesen zu haben 
glaube, dass die ganze Uranlage des centralen Nervensystems ein 
Derivat der weissen Hemisphäre des Eies sei... so bleibt es denkbar, 
dass die vorderen Teile der Markanlage, die dem Gehirn und möglich- 
erweise sogar dem oberen Teil des Riickenmarks entsprechen, sich 
in der schwarzen Hemisphäre bilden”. 
The controversy between Roux (1888) and Scuuurze (1887) is well 
known, the former of whom let the dorsal lip of the blastopore 
move over the white half of the egg through no less than 170—180°, so 
that the medullary canal consequently originated entirely on the white 
half, while Scnuurze on the other hand declared. all displacement of 
the blastopore border to be illusory and ascribed it to a rotation of 
the egg, so that it would just be on the black hemisphere that the 
medullary canal originates. Only on this point they agreed, but as 
we shall see, erroneously, that the egg axis afterwards has a 
dorsoventral direction. The place where the dorsal lip is first 
noticed is according to Rovx the rostral, according to ScHurrze the 
caudal end of the embryo. Brerraccmint (1899) and Hertwie (1902) 
took the side of Roux, Lworr (1894) that of Scnuurze. Among later 
investigators however, the opinion begins to prevail, that neither of 
the two conceptions mentioned is correct, but that the embryo is 
formed partly on the white, partly on the black hemisphere and that 
consequently the egg axis is not perpendicular to the longitudinal 
axis of the embryo but has more or less the same direction. This 
view was first put forth by Assurron (1895) and after him by Kopscu 
(1900), according to whom the egg axis lies in the embryo from a 
ventral point in front to a dorsal point behind. If ScnuLrze was of 
opinion that the formative material of the embryo lies entirely in front 
