GASTRULATION IN THE PIGEONS EGG. 259 



free edge of the posterior margin of the blastoderm. This inter- 

 pretation is in harmony with what is known to occur in the gas- 

 trulation of other forms, especially the fish. Thus Agassiz and 

 Whitman ('84) state that in Ctenolabrus " there is a plain rolling 

 under, or involution, as an initiatory step in the formation of the 

 ring," but they believe that it is more correct to describe the 

 process " as an ingrowth, due both to a rapid multiplication of 

 the cells, and also to the centrifugal expansion of the ectoderm." 

 There are certain differences between theteleost and pigeon blasto- 

 derms which, in this connection, must not be overlooked. Thus 

 at the time of invagination the teleost blastoderm is three or four 

 cells thick, and the epidermal layer of the ectoderm takes no 

 part in the involution. On the other hand, the pigeon blasto- 

 derm is approximately but one cell thick at the posterior margin 

 where invagination occurs, and hence all the cells of this margin 

 participate in the involution. The interpretation of sections cer- 

 tainly supports this view, but the appearance of sections is often 

 misleading. Two other sources of evidence are much more 

 convincing. In the first place, careful measurements show that 

 previous to and following gastrulation the blastoderm is nearly 

 circular, but during the period of invagination the antero-pos- 

 terior diameter is always shorter than the transverse diameter. 

 This is exactly what one would expect if the posterior margin 

 turns under instead of growing out over the yolk. In the second 

 place an injury made on the posterior margin of the blastoderm 

 during the early stages of invagination is found, upon further in- 

 cubation, in the gut-entoderm, that is, it has been carried down 

 under the blastoderm. 



There occurs simultaneously with the turning under of the free 

 edge a rapid thickening in the region of invagination, that is, on 

 the posterior border where the upper layer turns under to be- 

 come continuous with the invaginated portion. This thickening 

 is not to be accounted for merely by a multiplication of cells in 

 situ, but is largely brought about by a movement of material to 

 the median axis from the lateral portions of the posterior margin 

 of the blastoderm. This shifting of material necessarily brings 

 about the approximation of the horns of the germ-wall, and in 

 thus approaching each other they finally meet, and thus close the 



