Gastrulation in the Pigeon's Egg. 101 



On the other hand, in the majority of blastoderms there is no 

 apparent reduction in the lip preceding the closing, but it remains 

 quite as thick as that of Fig. 37, even after being entirely enclosed 

 Avithin the zone of junction. This can be made out from a series 

 of transverse sections of the blastoderm shown in Fig. XIII. Thus 

 in the section taken through xx' the entoderm is at least five cells 

 thick, and passes over gradually into the region of the zone of 

 junction (Fig, 22). Above the entoderm a distinct ectoderm is 

 differentiated, but slightly posterior to this it can no longer be 

 distinguished, and still farther back is found the zone of junction, 

 which completely encircles the blastoderm (Fig. XIII, Z). Ante- 

 riorly the entoderm rapidly thins out, the cells being arranged in 

 groups (Fig. 23), which become less and less thick, finally disap- 

 pearing altogether, so that only a few cells are found (Fig. 24). 



Interruption of the Posterior Zone of Junction. — We are now in 

 a position to consider the interruption of the zone of junction. It 

 was stated above that the interruption was associated with the degen- 

 eration of the periblastic nuclei in the region of the posterior zone. 

 Abundant evidence is found for this statement in the study of 

 any egg taken either just before or during gastrulation. Thus in 

 Figs. 8-13 is shown a series of nuclei in various stages of degenera- 

 tion. The first indication of the breaking down process is found 

 in the increase in the size of the nucleus. In this condition the 

 nuclei do one of two things. In some cases, they stain intensely 

 (Fig.* 8) and apparently the nuclear membrane breaks down directly, 

 leaving the chromatin lying free within the yolk (Fig. XIV, A). 

 In the large majority of cases, however, they continue to increase 

 in size and at the same time their capacity for stains gradually dimin- 

 ishes, until it is difficult to study them at all after the use of hema- 

 toxylin. When they have increased to a volume equal to many times 

 that of an average normal nucleus, they begin to divide (rarely into 

 equal parts — as in Fig. 10), or rather portions are pinched off 

 from the sides of the nucleus (Figs. 9 and 11) — the process continu- 

 ing until the entire nucleus is reduced to small fragments (Figs. 12 

 and 13). Finally one sees among the yolk spherules only clear 

 spaces, which indicate the places previously occupied by these frag- 

 menting nuclei. 



