Gastrulation in the Pigeon's Egg. 107 



process of digestion, by which the blastoderm is securing its nourish- 

 ment, and is doubtless similar to the phenomena of degeneration or 

 fragmentation of the yolk that has been described by many workers 

 on practically all of the vertebrate ova (Barfurth in the Teleosts; 

 Dean in the Chimaeroids ; Stahl in the Reptiles ; Ruge and Born in 

 the Amphibians ; Pf liiger in the Mammals ; Brunn and others in 

 the Birds). 



There are a few small cells within the cavity that are still to be 

 accounted for (Fig. 53). These may come from two sources: either 

 they are lower segmentation cells that have failed to get into the 

 differentiating ectoderm, or they are wandering entoderm cells 

 (Gasser, '82). If they come from the latter source and are later 

 taken into the entoderm, no further consideration is necessary ; but 

 if they are to be regarded as coming from the former source, we 

 may justly ask. Why is it that' when an egg is fixed in an inverted 

 position during the differentiation of the ectoderm, no cells are found 

 in the cavity ? Whatever be their source, they are too insignificant 

 in numbers to be of any great importance. 



So far, we have considered the question of whether or not the 

 invaginated or gut-entoderm receives cells from the anterior or lateral 

 parts of the germ-wall, and on the whole the evidence favors the 

 negative ; but in regard to the relation of the entoderm to the posterior 

 germ-wall, further considerations arc necessary. It was stated above 

 that as a result of the manner in which the blastopore closes, the 

 dorsal lip comes to lie within the margin of the blastoderm. Hence, 

 in longitudinal sections, the entoderm, while ending anteriorly with 

 a free margin (Fig. 42), appears to arise directly from the posterior 

 germ-wall. This apparent union of the entoderm with the posterior 

 wall is only secondary, and the greater part of the mass of cells 

 here l)elongs to the dorsal lip. This is most obvious immediately 

 after the closing of the blastopore, when the ectoderm is not differen- 

 tiated from the underlying mass (Fig. 26). 



Although Balfour ('82) and many other investigators, working 

 on the uniycubated hen's eggs, have noted that the entoderm is in- 

 complete anteriorly and united to the germ-wall posteriorly, yet 

 Nowack ('02) was the first to clearly state that the entoderm was 



