PRINCIPAL AXIS OF SYMMETRY IN THE BIRD S EGG. 337 



o i per cent. 



45 26 per cent. 



90 50 per cent. 



135 8 per cent. 



180 3 per cent. 



Inversions 12 per cent. 



(see p. 347 below). 



Dalton also took cognizance of the relation between the ovum 

 and the embryo for he suggests that the position of the embryo 

 on the yolk may be due to the way in which segmentation takes 

 place in the "cicatricula"- referring to Kolliker's suggestion 

 regarding the eccentricity of cleavage. He then remarks that 

 variation in the relation of the embryo to the shell may be due 

 to the manner in which the yolk is received and transported 

 through the oviduct. He at least, had not failed to read v. Baer. 

 Duval (1884) who does not seem to have ever seen "Ueber 

 die. Entwickelungsgeschichte der Thiere" and who speaks of 

 "la loi enoncee par Balfour," was the first to make practical use 

 of the axis angle. He saw that it afforded a means of orienting 

 stages previous to the appearance of the primitive streak and 

 accordingly made a "petite statistique" upon 166 hen's eggs 

 incubated 39 hours. He observed the angle between embryo 

 and shell and to judge from his diagrams grouped them in the 

 following classes : 



90 124 eggs 76 per cent. 



circa 65 13 8 per cent. 



circa ii o 26 " 1 6 per cent. 



180 2 i per cent. 



(Inversion) 270 i egg 0.6 per cent. 



The large number of cases at or very near 90 may be due partly 

 to the personal equation for which there are many loopholes 

 in his method of measurement, partly perhaps to his having 

 obtained his eggs from a small flock of hens. 



Fere (1900) published the most extensive data that have 

 appeared on the hen's egg, in connection with his demonstration 

 that such experimental eggs as develop normal embryos have no 

 more variation in the axis angles than the controls do. His 

 results are tabulated as percentages of "deviations." He con- 

 siders all cases which show a variation of 45 or more from the 

 "norm," as "deviations." Expressed in my terms this means 

 there is a normal variation of 90, from 45 to 135 which corre- 



