GASTRULATION IN THE PIGEON'S EGG. 257 



their number and size in the anterior half of the blastoderm (Fig. 

 3), where they are about seven layers deep. However, as one 

 passes from the anterior to the posterior margin there is a grad- 

 ual change in depth from seven cells to one. There are also 

 found in the anterior region large yolk masses (Fig. 3, M), which 

 arise from the floor of the segmentation cavity. At this stage 

 they are not limited to this region, but occasionally are found 

 in the posterior half (Fig. 4, M), where the disappearance of the 

 germ-wall is one of the most characteristic features. This inter- 

 ruption of the germ-wall goes hand and hand with the thinning 

 out, which is rapidly establishing a one-layered condition of the 

 blastoderm. In other words the phenomenon of thinning-out is 

 nothing more nor less than the crowding of the cells of the seg- 

 mented disc into a single layer. It is evident that this must 

 result in a rapid centrifugal expansion of the blastoderm. That 

 this is actually the case is shown by measurements. Thus at 

 twenty hours after fertilization the average diameter of the blasto- 

 derm is 1.9 1 5 mm., while at thirty hours it is 2.573 mm - in fact 

 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 out is at its maximum. One 

 would not be justified, however, in saying that this entire expan- 

 sion is brought about by the thinning out, for according to Miss 

 Blount's interpretation the germ-wall is also contributing ma- 

 terially to this increase. 



In the posterior third of this blastoderm the single layer is 

 almost complete ; still, at places, some of the few cells yet re- 

 maining in the segmentation cavity can be seen apparently in the 

 act of crowding up into the single layer (Fig. 4, X). Whether 

 or not, in all cases these remaining cells eventually succeed in 

 getting into the upper layer, approximately above where they are 

 situated is not clear. They do in the majority of blastoderms, 

 but I have some few series in which they seem to migrate an- 

 teriorly and to the sides, where the last stages of thinning out 

 occur. In either case they take no part in the formation of the 

 gut-entoderm. 



In Fig. 5 is shown a reconstruction from sections of the blasto- 

 derm represented in Figs. 3 and 4. The germ-wall does not 



