Section VI 



EMBRYOGENESIS: PROGRESSIVE 

 DIFFERENTIATION 



CHAPTER 1 



Amphibians 



J. HOLTFRETER AND V. HAMBURGER 



EGG ORGANIZATION AND 

 DETERMINATION OF AXES 



EGG STRUCTURE 



Although the uncleaved amphibian egg has 

 been used extensively for experimentation 

 since the beginnings of experimental em- 

 bryology, our information concerning its 

 organization is very limited (see Fankhauser, 

 '48). External signs of structure are few: 

 the pigment concentration in the cortical 

 layer of the animal hemisphere, the yolk 

 concentration in the vegetal hemisphere, 

 and a clear area near the animal pole, indi- 

 cating the location of the egg nucleus and 

 of the polar bodies. In some species, a dorso- 

 ventral polarization and the plane of sym- 

 metry can be detected before fertilization 

 (Pasteels, '37a; Banki, '27b). The descrip- 

 tions of the internal egg organization indi- 

 cate regional differences in the size and 

 concentration of yolk platelets and of pig- 

 ment granules, and a cytoplasmic organiza- 

 tion into a more fluid inner plasmasol and 

 a more gelated outer cortex; the latter is 

 covered by a cell membrane and a protective 

 surface coat (Holtfreter, '43a). These com- 

 ponents are arranged in a complicated pat- 

 tern and not in the fashion of simple gradi- 

 ents, as is often postulated for the sake of 

 theoretical simplifications (Fig. 75a) (Ancel 

 and Vintemberger, '48; Pasteels, '51). Banki 

 ('27a) and Lehmann ('42a) have described 

 in the eggs of several urodeles an equatorial 

 ring of specialized, subcortical "marginal 

 plasma" which may be the precursor of the 

 marginal zone of tihe gastrula. 



Shortly after fertilization, the whole egg 

 lindergoes a rotation within its jelly coats, 

 whereby its animal-vegetal axis assumes a 

 vertical position ("rotation of orientation," 

 Ancel and Vintemberger, '48). In some 

 anurans and urodeles, dorsoventral polarity 

 and bilateral symmetry become visible 

 shortly before the first cleavage with the 

 appearance of the gray crescent, which is a 

 sickle-shaped grayish area at the margin 

 of dark and white material. Observations and 

 vital-staining experiments have confirmed 

 the contention of Roux ('03) and others that 

 the gray crescent demarcates the area above 

 the future blastopore, and thus the dorsal 

 side of the future embryo whose head will 

 be located in the region of the animal pole. 

 As a rule, the plane bisecting the gray cres- 

 cent becomes the median plane of the em- 

 bryo (Vogt, '26a; Banki, '27a; Pasteels, '37a; 

 see, however, p. 233). 



The formation of the gray crescent is a 

 visible sign of more fvmdamental changes in 

 the egg organization which begin long before 

 the gray crescent appears. In eggs with a 

 clearly demarcated ventral pigment border 

 (for instance, Rana fusca), one observes a 

 rotation of the pigment cap. Its border shifts 

 from its original horizontal position (m-n, 

 Fig. 75a) to an obliqvie position mi-m. Fig. 

 75b), and the animal pole moves downward 

 (from an, Fig. 75a, to am. Fig. 75 b), its 

 final inclination amounting to 15 to 30 de- 

 grees. The gray crescent appears on the side 

 at which the margin of the pigment cap rises 

 above the equator, and its median plane 

 coincides with the meridian along which the 



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