232 



Embryogenesis: Progressive Differentiation 



symmetrization," and therefore also with the 

 median plane of the gray crescent. The latter 

 originates around that point of the border 

 between dark and white material which was 

 highest on the inclined egg. Since one can 

 mount the egg in any desired position, any 

 meridian can be chosen as the median plane 

 and any point on the circumference of the 

 white vegetal field as the dorsal side of the 

 future embryo. 



In an extensive re-investigation of the 

 classic inversion experiment of Schultze 

 (1894), Penners and Schleip ('28a,b) have 

 shown that the plane of symmetry can be 

 altered even after the appearance of the 

 gray crescent. Eggs of R. fusca in the 2-cell 

 stage were compressed between glass plates, 



Fig. 76. Diagram of rotation experiment to deter- 

 mine the median plane. An unfertilized frog's egg 

 in its jelly membranes is mounted on a glass plate; 

 the egg axis forms an angle of 135 degrees with the 

 vertical (after Ancel-Vintemberger, '48, Fig. 1). 



the positions of their gray crescents were 

 marked on the glass plate, and the fixed eggs 

 were then kept in an upside-down position 

 for several hours. As a result, the yolk and 

 the cytoplasm underwent a complete rear- 

 rangement. Several significant points were 

 established: (1) The blastopore, or blasto- 

 pores (in the case of duplications) were 

 formed invariably at the border of dark and 

 white material. (2) The site of the blasto- 

 pores was preferentially at, or near, the 

 center of the original gray crescent. (3) The 

 invaginations and their direction were in- 

 fluenced by the abnormal yolk distribution. 

 As a result, the median plane or planes of 

 the embryos could be at any angle to the 

 original plane of symmetry. Similar experi- 

 ments of Pasteels ('38, '39a) confirmed these 

 results for Rana esculenta and Discoglossus. 

 Of particular significance is the second point. 

 It implies that the inversion did not dislocate 

 the original gray crescent. This led the 

 authors to suggest that a factor which deter- 

 mines the site of invagination and thus the 

 dorsal side may reside in the cortex. It is 



relatively stable at a stage when the sym- 

 metry plane is still labile. In other words, 

 the experiment has succeeded in disengaging 

 the factors for dorsalization from the factors 

 for symmetrizations: The former are stabil- 

 ized first; they determine a preferential site 

 for invagination but not necessarily its direc- 

 tion. The direction of invagination, and thus 

 the cephalocaudal polarity and bilateral sym- 

 metry of the embryo seem to depend on the 

 distribution of yolk. It is possible that the 

 yolk plays a passive role in this respect, serv- 

 ing merely as a support to direct cytoplasmic 

 shifting, similar to the condition in an un- 

 fertilized, aged egg kept in hypertonic salt 

 solution. There the pigmented cortex can "in- 

 vaginate" by moving inside, around a plug 

 of vegetal yolk (Holtfreter, '43b). 



Experiments with Localized Insemination. 

 The localized insemination of the frog's egg 

 by Newport (1854), which was probably the 

 first analytical experiment in embryology, 

 and a similar experiment by Roux (1885, 

 1887), which was conceived independently, 

 demonstrated a close correlation between 

 the sperm entrance point and the plane of 

 first cleavage which latter passed through 

 animal pole, vegetal pole and sperm entrance 

 point. Although the contention that the first 

 cleavage plane coincides invariably with 

 the median plane of the embryo was not 

 borne out by later investigations (Vogt, '26a; 

 Banki, '27b; and others), the correlation be- 

 tween sperm entrance point and median 

 plane was confirmed by Ancel and Vintem- 

 berger ('48) and others. It is likely that the 

 sperm actually determines bilateral sym- 

 metry under natural conditions, at least in 

 some species (see Pasteels, '37a). However, 

 its role is not universal or indispensable. For 

 instance, normal symmetry relations establish 

 themselves in polyspermic species of urodeles 

 (see Fankhauser, '25) and after experiment- 

 ally induced parthenogenesis (A. Brachet, 

 '11, and others). 



All observers, then, agree that the dorso- 

 ventrality and bilateral symmetry of the 

 uncleaved egg are highly labile and readily 

 modifiable by gravity and localized insemi- 

 nation. The former is stabilized first; the 

 latter remains labile at least up to the 2-cell 

 stage. 



INITIAL STEPS OF ORGAN 

 DETERMINATION 



The axial determination is only one as- 

 pect of egg organization. The early deter- 

 mination of organ patterns is of equal signi- 



