Gastruliitiun in the Pigeon's Egg. 79 



In connection with this stage (Fig. 1) it remains only to call 

 attention once more to the thinness of the blastodisc above the seg- 

 mentation cavity (sc). While there is some evidence in favor of 

 the view that this thin condition existed from an early cleavage 

 stage, yet, in the light of subsequent events, it lends itself to another 

 interpretation, namely, that it is the beginning of a thinning-out 

 process which will eventually succeed in producing a one-layered 

 condition of the segmentation cells. In other words, all the cells 

 of the segmented disc finally arrange themselves into an epithelial- 

 like structure, the primary ectoderm. This interpretation for Fig. 

 1 receives support also from a study of several slightly younger 

 stages, which show the disc to be from three to five cells thick in 

 the central region. 



As we have seen in surface views, the thinning-out does not begin 

 exactly in the center of the disc (Fig. 1), but slightly posterior 

 to this place, and then spreads in all directions but with more rapidity 

 toward the posterior margin. This thinning-out evidently brings 

 about a rapid centrifugal expansion of the disc, for there is no other 

 period in the early history of the blastoderm in which there is such 

 a rapid increase in the surface area, as occurs during the time when 

 the thinning is at its maximum. 



Coincident with the thinning-out, but not connected with it, another 

 important process makes its appearance, that of the interruption of 

 the posterior zone of junction. This interruption is associated with 

 the degeneration of the periblastic nuclei beneath the zone of junction. 

 The presentation of the facts upon which this conclusion is based 

 must be deferred until more advanced stages have been described. 



Let us consider next a scries in which the progress of the thinning- 

 out as well as that of the interruption of the zone of junction can 

 clearly be seen. A median longitudinal section of such a stage is 

 shown in Fig. 29, PI. IV. The blastoderm from which this photo- 

 graph was made is considerably in advance of that of Fig. 1 and 

 would correspond to stage D, Fig.V. In addition to the progress 

 made in the thinning-out and the interruption of the posterior zone 

 of junction, the more important changes are (1) the great increase 

 in the number of cells, and (2) the extension of the segmentation 



