ORIGINS OF EMBRYONIC PATTERNS 685 



plane of the embryo existed. When the gray crescent was discovered, 

 such a relation seemed all the more probable, since the dorsiventrality 

 indicated by the crescent was believed to become visible only after fer- 

 tilization. Experimentally localized application of sperm to the egg and 

 histological studies of fertilization showed a high frequency of coincidence 

 or near coincidence of median embryonic plane and plane determined by 

 point of entrance of sperm or its path toward the female pronucleus 

 after its entrance. Jenkinson believed, however, that other factors, 

 gravity or light, might play a part.'^ The view that the spermatozoon 

 determines or is an important factor in localizing the median plane found 

 wide acceptance until the question was again opened by further investi- 

 gation. Polyspermic eggs of Rana fusca were found to develop a normal 

 gray crescent (A. Brachet, 1910a, b), and in parthenogenetic development 

 induced by puncture a normal gray crescent and dorsiventrality develop 

 without relation to the meridian of puncture (A. Brachet, 1911a, &; 1927). 

 Moreover, evidence that the crescent appears in some amphibian eggs 

 before fertilization was presented by Vogt (19266, 1928^)). Recent attempts 

 to throw further light on the question indicate that the median plane 

 may be determined independently of the meridian of sperm entrance.'^ 

 According to Tung, entrance point of sperm is ventral, with rare ex- 

 ceptions, in normal development; but in dispermic eggs a definite rela- 

 tion between dorsiventrality and sperm entrance points does not appear. 

 Since gray crescent and dorsiventrality appear in polyspermic and in 

 parthenogenetic eggs, Brachet concluded that a labile dorsiventrality is 

 present in the unfertihzed egg but may be altered by the spermatozoon. 

 Another possibiHty, suggested by Spemann and Falkenberg (1919) and 

 regarded favorably by Dalcq (1935), is that the ventral region of a pre- 

 determined dorsiventrality is a preferential region of sperm entrance. 

 However, the earlier view that dorsiventrahty is epigenetically deter- 

 mined by the sperm still finds support (Wintrebert, 1933a, h; 1934). At 

 present it appears beyond question that a normal dorsiventrality and 

 crescent may appear in parthenogenetic and polyspermic eggs of various 

 amphibians. Moreover, the fact that, even when the fertihzation meridian 

 is in the ventral region, it is by no means always in the median plane and 

 may be at some distance from it indicates that other factors than the 

 sperm are concerned in determining dorsiventrality; but it does not ex- 



=« Newport, Ellis, and Forbes, 1854; Roux, 1887; A. Brachet, 1903, 1904; Jenkinson, 1906(2, 

 1909. 



^9 Banki, 1927; Wcigmann, 1927; Gilchrist, 1932; Tung, 1933; Pastcels, ig^h. 



