688 PATTERNS AND PROBLEMS OF DEVELOPMENT 



of some forms (p. 149), but how these differences originate is not known. 

 If they are expressions of axiate pattern, they indicate the radius or region 

 of initiation of axiate development, and bilaterality is apparently a sec- 

 ondary result of a longitudinal and dorsiventral differential in the develop- 

 ing region. This region is not the only one capable of forming an embryo, 

 as experiment has shown (pp. 521-22), but is probably living a little more 

 rapidly than other parts, so that certain events occur in it earlier than 

 elsewhere, and its dominance determines the course of development. How 

 it is localized is still to be discovered. 



The same question, how the longitudinal embryonic axis and the median 

 plane are determined, arises with regard to reptiles and birds. Data on 

 reptile eggs are few: the embryonic axis of the gecko is said to be ap- 

 proximately at right angles to the long axis of the egg (Will, 1893). In 

 the chick the embryonic axis is more or less nearly at right angles to the 

 long axis of the egg shell; and, with the pointed end to the right, the 

 anterior end of the embryo is usually directed away from the observer.-'' 

 This orientation, however, is by no means constant; deviation in either 

 direction is frequent but usually not great, though inverted orientation 

 sometimes occurs. ^^ The pigeon's egg shows a somewhat similar relation 

 between long axis of the embryo and of the whole egg, but the modal 

 angle between the two is about 70°, with variation from 8° to 135°, and 

 inversion is rare.^^ 



The early oocyte of the pigeon, according to Bartelmez, exhibits bi- 

 laterality. A polar axis is indicated by the eccentric position of the nu- 

 cleus nearer one pole, the future apical pole, and by positions of cyto- 

 plasmic granules and yolk nucleus. This polar axis is "not infrequently" 

 vertical to the surface of the ovary, with the apical pole attached. A 

 second axis, the long axis of the oocyte, is also indicated, and bilaterahty 

 results from position of the nucleus nearer one pole of the long axis, that 

 is, only one plane divides the oocyte into symmetrical halves. Whether 



31 Von Baer's rule; von Baer, 1828. 



s^Duval, 1884, found the two axes vertical to each other in 75 per cent; Kopsch, 1927, in 

 :iS per cent; and other figures range between these — e.g., Butler, 1935, 50 per cent. According 

 to Butler, deviation up to 45° to the right is 33.1 per cent; to the left, 10.6 per cent; more 

 than 45° to the right and left, 2.5 per cent each. See also Fere, 1900; Rabaud, 1908. Percent- 

 ages of inversions given by different authors vary widely: J. C. Dalton, 1881, 12 per cent; 

 Duval, 1884, 0.6 per cent; Bartelmez, 191 8, ^;i per cent. 



" Bartelmez, 1912, 1918. See also Blount, 1909; Patterson, 1909. Bartelmez reports 0.67 

 per cent of inversion in one lot of 600 eggs and 2 per cent, all in eggs of two of some 90 birds 

 with a total of 400 eggs. 



