Amphibians 



231 



animal pole has moved. Obviously, we are 

 dealing with a second rotation-phenomenon 

 ("rotation of symmetrization," Ancel and 

 Vintemberger, '48). It has been generally 

 assumed that this rotation involves the en- 

 tire egg, leading to a redistribution of heavy 

 and light materials and to a shift of the 

 center of gravity. However, Ancel and Vin- 

 temberger (since '32, see '48) have demon- 

 strated for R. fusca, by means of electro- 

 cautery marks, that only the egg surface is 

 involved. The cortex glides over the heavy 

 materials of the center, which later remain 



Interest in these problems has been revived 

 more recently. 



Some authors contend that a bilateral 

 symmetry pre-exists in all unfertilized am- 

 phibian eggs and not only in those in which 

 it is actually visible. The appearance of 

 normal symmetry relations in eggs which 

 have been activated by chemical agents or 

 by electrical shock may be cited in support 

 of this contention. Such an inner organiza- 

 tion would originate in the ovary. 



Numerous investigations have shown that 

 whatever intrinsic organization pre-exists, 



a b 



Fig. 75. Diagram of the structure of an unfertilized frog's egg (a) before and (b) after rotation of sym- 

 metrization (after Ancel and Vintemberger, '48, Fig. 35). a. Animal pigmented cytoplasm with small yolk 

 granules; an, anu position of animal pole before and after rotation; b, central pigmented yolk area; c, vegetal 

 unpigmented area with large yolk granules; m-n, m.\-n\, ventral margin of pigment before and after 

 rotation. 



static except for a slight shift in the marginal 

 plasma (Fig. 75), 



The origin of the gray crescent is not 

 clearly understood. According to the above 

 avithors, it originates in anurans in the fol- 

 lowing way: on the future dorsal side, only 

 the cortical plasma carrying pigment moves 

 upward, but the more deeply located pig- 

 ment is left behind. The latter is thus over- 

 laid by a thin layer of unpigmented plasma, 

 resulting in the gray hue of the crescent. 

 According to Roux ('03), pigment is dis- 

 persed into the interior. Vital-staining ex- 

 periments of Banki ('27a) indicate that in 

 urodeles cortical or subcortical materials 

 spread also towards the vegetal pole during 

 the formation of the gray crescent. The mat- 

 ter requires fvirther elucidation. 



AXIAL DETERMINATION 



The analysis of the factors involved in 

 axial determination has been a favorite ob- 

 ject of experimentation in the first period of 

 experimental embryology (Roux, Pfliiger, 

 Born, Schultze, and others; see Morgan, '27). 



it is highly labile and modifiable; in fact, 

 it is possible to impose on the uncleaved egg 

 a median plane in any desired meridian. The 

 two classical methods by which this can be 

 accomplished are {a) the use of gravitational 

 force (rotation, centrifugation, fixation in 

 abnormal positions) and {b) localized in- 

 semination. 



Effects of Gravitational Force. The follow- 

 ing experiment of Ancel and Vintemberger 

 ('48) gives an impressive demonstration of 

 the lability of the internal organization. Un- 

 fertilized eggs {R. fusca) are mounted on a 

 glass plate, with the animal-vegetal axis 

 inclined 135 degrees (Fig. 7Q). Following 

 parthenogenetic activation by electric shock, 

 water is added, whereupon the egg under- 

 goes its "rotation of orientation." Marking 

 experiments have shown that the plane in 

 which the animal-vegetal axis moves during 

 this rotation coincides invariably with the 

 median plane of the future embryo. In 

 other words, the meridian in which the ani- 

 mal pole moves during the "rotation of 

 orientation" coincides with the one in which 

 it moves during the subsequent "rotation of 



