128 
which purpose ZEISS has even constructed a special ap- 
paratus for HERTWIG (1906, p. 740), we run the risk that 
the egg, being prevented from assuming its natural position 
of equilibrium, does not develop in a normal way. 
Different views on movement of blastopore border. — Asa 
consequence, the opinions on the movement of the blastopore 
border have been very divergent. The oldest view is that 
the black hemisphere becomes the dorsal part of the 
embryo, so that the egg axis lies dorsoventrally. As is well 
known, PFLÜüGER (1883) first pointed out that the blasto- 
pore moves forward over more than 90° from the point 
„where the dorsal lip first appears, from which PFLÜGER 
concluded that the foundation of the nervous system 
originates on the white hemisphere. He added however: 
“Um nicht missverstanden zu werden, möchte ich hervor- 
heben, wie ich keineswegs bewiesen zu haben glaube, dass 
die ganze Uranlage des centralen Nervensystems ein Det ivat 
der weissen Hemisphäre des Eies sei .... so bleibt es 
denkbar, dass die vorderen Teile der Markanlage, die dem 
Gehirn und möglicherweise sogar dem oberen Teil des Rücken- 
marks entsprechen, sich in der schwarzen Hemisphäre bilden”’. 
The controversy between ROUX (1888) and SCHULTZE 
(1887) is well known. The former concluded that the dorsal 
lip of the blastopore moves over the white half of the egg 
through no less than 170°—180°, so that the medullary 
plate consequently originates entirely on the white half. 
SCHULTZE, on the other hand, declared all displacement of 
the blastopore border to be imaginary and ascribed it to the 
rotation of the egg, so that it would be just on the black hemt- 
sphere that the medullary canal originates (cf. fig 34). They 
agreed, erroneously, as we shall see later, only on the point 
that the egg axis afterwards has a dorsoventral direction. 
The place where the dorsal lip is first noticed is according 
to ROUX the rostral, according to SCHULTZE the caudal, end 
of the embryo. HERTWIG (1892) and BERTACCHINI (1899) took 
the side of ROUX, LWOFF (1894) that of SCHULTZE. Among 
later investigators, however, the opinion begins to gain 
ground that neither of the two conceptions mentioned ÍS 
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 ASSHETON (1894) and EYCLESHYMER 
