The Early Development of the Pigeon's Egg. 37 



Kolliker ('76, Fig. 16) gives a figure of a lien's egg in the two- 

 celled stage, in which the first cleavage plane is transverse to the 

 longitudinal axis of the embryo. He describes it as follows: "Die 

 Keimscheibe war weiss, nahezu 3 mm. gross, von einem schmalen 

 dunklen Ilofe umgeben und durch eine mittlcre bogenformigo 

 Furche unvollstandig in zwei Halften geschieden." The "dunklen 

 Hofe" is, I think, the periblast. 



The Four-celled Stage. — I present a series of figures (18 to 26) 

 of pigeon eggs in approximately the four-celled stage. The orien- 

 tation is the same as indicated in Fig. 1. (Compare Fig. 25 with 

 Kolliker ('76) Fig. 17.) In these two figures, the cleavage planes 

 are in exactly the same relation to the chalazal axis, but in the 



Fig. 17. — A living pigeou egg. When tlie egg was obtained, the cleavage 

 furrow 1+2 was the only one in view. 3 appeared at 3.30 a. m., and 4 

 came at 3.3.j a. ni. The egg was obtained abont five hours after fertilization. 



hen's egg the axis of the embryo is at right angles to the chalaza, 

 and in the pigeon's egg the embryo is diagonal (Fig. 1). A com- 

 parison of all these figures suggests that the early cleavage planes 

 of the pigeon's egg bear no constant relation to the axis of the 

 future embryo. 



Asymmetry. — These figures also suggest what is confirmed in later 

 stages (see Figs. 27 to 40 and see photographs), that the cleavage 

 is not always excentric, as represented by Kolliker, "Die Furchung 

 geht immer asymmetrisch vor sicli, so dass oline Ausnahme die eine 

 Halfte der Keimscheibe in der Zerklilftung dor andern voran ist 

 und die Haui^tmasse der Kugeln und ebenso die kleineren Segmente 



