BILATERALITY OF THE PIGEON's EGG 299 



evidence in the ovarian history which independently led to this 

 conclusion. 



After ovulation and fertilization, i.e., between the second matur- 

 ation and the first cleavage, the whole blastodisc gives evidence 

 of its bilateral organization by a progressive change in shape 

 which can be followed in the living egg. At the time of ovula- 

 tion, which usually takes place when the second maturation spindle 

 is at metaphase, the segmental disc and periblastic zones are 

 circular, so far as can be seen. After fertilization they gradually 

 become elliptical, so that the antero-posterior diameter is shorter 

 than the right-and-left, as may be seen in figures 35, 36 and 43. 

 A periblastic ring which appears at this time gives further evidence 

 of the bilateral organization of the blastodisc. The most strik- 

 ing manifestation of bilaterality, however, is within the segmen- 

 tal disc itself. The granules forming the 'wedge' are gradually 

 moved from the anterior side of the segmental disc and form a 

 crescent around its posterior margin. This movement can be 

 followed in the living egg and will appear clearly if figures 40 

 and 41 be compared. The changes that take place between 

 these two stages have been worked out in detail and will be pub- 

 lished shortly. 



VI. AXIS ANGLES 



The long {or chalazal) and embryonic axes 



It has been seen that in the incubated egg, two axesof bilat- 

 erality can be distinguished: 



1. The embryonic axis. 



2. The axis of bilaterality of the ovum as a whole, which is 

 defined by: (a) the long axis of the ovum; (b) the position of 

 the latebra nearer one end of the long axis. 



The relation between these axes is shown in diagram I, and 

 its development is illustrated in diagram II. The essential fea- 

 ture of this relation is that when a definite end of the long axis 

 is held to the right, the head of the embryo is away from the 

 observer. Hitherto this relation has been described as existing 

 between the embryonic and chalazal axes, but it was shown, 

 (p. 295), that the long axis is the primary one, since it determines 



