GASTRULATION AND EARLIEST DEVELOPMENT 177 



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. 



Dijferent view^ 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 dorso/entrally. As is well 

 known. PPLuGER (1883j fir.>t pointed out that the blasto- 

 pore moves forward over more than 90° from the point 

 where the dorsal lip first appears, from which PfLuGER 

 concluded that the foundation of the nervous system 

 originates on the white hemisphere. He added however: 

 "Um nicht missverstanden zu werden, mochte ich hervor- 

 heben, wie ich keineswegs bewiesen zu haben glaube, dass 

 die ganze Uranlage des centralen Nervensystems ein Deiivat 

 der weissen Hemisphere des Eies sei . . . . so bleibt es 

 denkbar, dass die vorderen Telle der Markanlage, die dem 

 Gehirn und moglicherweise sogar dem oberen Teil des Riicken- 

 marks entsprechen, sich in der schwarzen Hemisphare 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 hemi- 

 sphere that the medullary canal originates (cf. fig 34). They 

 agreed, erroneously, as we shall see later, only on the point 

 that tiie egg axis afterwards has a dorsoventral direction. 

 The place wherf^ the dorsal lip is first noticed is according 

 to Roux the rostial, according to SCHULTZE the caudal, end 

 of the embryo. HERTWIG (1892) and BeRTACCHINI ( 1 899) 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 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 ASSHETON (1894) and EycleshyMER 



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