GASTRULATION IN THE PIGEON'S EGG. 255 



lation, I found the close of segmentation to be the most advan- 

 tageous time, for it is shortly after this period that the first direct 

 steps leading up to invagination occur. At the close of segmen- 

 tation the disc is three or four cells deep, except at the extreme 

 margin where it gradually diminishes to a thickness of one or 

 two cells. Beneath and external to the marginal cells of the disc, 

 yolk or " periblastic " nuclei are present. According to Miss 

 Blount ('07) these nuclei segregate about themselves the neigh- 

 boring protoplasm, and later, cell walls appearing are added to 

 the disc, thus contributing to its extension. Waldeyer ('69), Hert- 

 wig ('99), and others have advanced similar views for the chick 

 and selachian, designating it supplementary cleavage. Where 

 these cells are being added to the disc a more or less syncytial 

 condition exists around the entire margin. This region consti- 

 tutes the germ-wall. 



Shortly after the period described above there occurs the first 

 direct step in the process of gastrulation. This is in the nature of 

 a thinning of the posterior part of the segmented disc. This proc- 

 ess begins, not at the extreme margin, but usually slightly pos- 

 terior to the center, and then spreads in all directions, but with 

 more rapidity towards the posterior margin. 1 The first stage of 

 this process is shown in Fig. 2. Slightly posterior to the center 

 and almost directly above the segmentation cavity, the disc is 

 but two cells deep, while in the region of the germ-wall it is four 

 deep. The characteristic features given above for the germ-wall 

 and the extension of the disc can also be made out from this fig- 

 ure. At this time the segmentation cavity is still very shallow, 

 but upon further progress of the thinning out it becomes much 

 more extensive, and may then be called the subgerminal cavity. 



As the thinning out progresses the germ-wall becomes inter- 

 rupted in the posterior region, as is shown in Fig. 4. The 

 blastoderm from which this drawing was made is much more ad- 

 vanced than that in Fig. 2, being about eleven hours older. The 

 changes occurring between these two stages, however, are grad- 

 ual and may be followed with comparative ease. In the first 

 place there is a very rapid division of cells, as is evidenced by 



1 In Torpedo ocellata Zeigler ('02) describes the thinning-out of the blastoderm 

 as beginning at the posterior and progressing anteriorly. 



