324 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



The remarkable fact is that the situation acquired in this way is still modifiable 

 for a certain lapse of time. Up until the fusion of the pronuclei, adequate mani- 

 pulations, consisting in reversing the egg and thus causing it to rotate by gravity 

 in a chosen direction (Fig. 30, a, b, p. 343), will change the plane of symmetry. 

 In this simple but clever experiment the main axis, from, pole to antipole, remains 

 unchanged, but the dorso-ventral axis can be displaced at will (Fig. 15). The 

 reason lies in a diffuse traction exerted by the chorion on the pellicle and the 

 associated cortical layer, through the viscous perivitelline fluid; this movement 

 stretches the yolk of the antipole which modifies the original yolk wall or produces 

 a new one. Thus, a reciprocal interaction exists between the cortical layer and the 

 underlying endoplasm. Until premitotic activity more or less fixates the situation, 

 the same general effect may be obtained by a process starting either in the endo- 

 plasm (normal course of events) or in the cortex (orientated rotation). 



What importance are we to conceed to these results? Are they simply experi- 

 mental findings, of limited interest because they concern only the amphibians, 

 or do they reveal of a more general mechanism at work in normal development? 

 This second possibility holds true at least for birds, especially for the pigeon. In 

 these eggs, the probable orientation of the dorso-ventral axis is given by the 

 Van Baer's rule. This prognosis is based on the form of the shell (round and 

 pointed end) which corresponds inside to a definite torsion of the chalaziae. The 

 torsion suggests that, during the time when the egg descends in the oviduct and 

 is gradually covered by the layers of albumen, it could be normally submitted 

 to a prolonged rotation somewhat similar to the conditions in amphibians. This 



Fig. 16. Cleavage and gastrulation in Camaesura anginna (Lacertilian). (a) The early blas- 

 toderm divided into approximately 6 blastomeres (good example of partial segmentation) ; 

 (b) the expanded blastoderm nearly covers the upper half of the yolk; in its center a large 

 area pellucida can be seen with aspects of yolk desintegration at the periphery and the em- 

 bryonic shield in the center. The immigration process, accompanied by medio-dorsal conver- 

 gence and axial, cephalo-caudal stretching is already nearly completed. The limit of the 

 homogeneous spreading mesoblast is perceptible outside the medullary plate; the trans- 

 verse fold of the brain is emerging, limited anteriorly by an amniotic groove (posterior to 

 the proamniotic crescent). The axis of the embryo is perpendicular to the axis of the egg, 

 as if v. Baer's rule for birds were valid for reptiles, which appears more and more likely 

 (see p. 325). Courtesy of Prof. J. Pasteels. 



