Gastriilatioii in the Pigeon's Egg. 97 



is doubtless to be accounted for by the fact that the teleostean embryo 

 is precociously formed, that is, as compared with the formation 

 of the avian embryo. 



The conditions at the anterior end of this section (Fig. 18) are 

 entirely different from those at the opposite end. First of all, the 

 differentiation of the ectoderm into a single layered epithelium is 

 not complete, for in many places the lower segmentation cells are 

 crowding upward against its under surface, although some of them 

 have sunk down into the cavity (s), but aside from these few the 

 subgerminal cavity is entirely free from nucleated cells anterior to 

 the fore end of the entoderm (e). There are found in the cavity 

 only large yolk masses, some of which are disintegrating (dm). 



The germ-w^all is not well differentiated in this section, but in 

 the sections to either side it is clearly defined. 



The region of overgrowth (o) is a wedge-shaped process extending 

 out from the zone of junction. The earliest stage in which this 

 region has been observed is illustrated in Fig. 27, and is characterized 

 by having no periblastic nuclei either beneath it or external to it, 

 and by having a fine granular area just beneath its under surface. ^^ 

 This region arises when the thinning-out is at its maximum and at 

 first is three or four cells thick, but later becomes reduced to a single 

 layer of cells (Fig. 28). 



The phyletic significance of this region is not clear. On the 

 one hand, it might be compared to the overhanging margin of the 

 selachian blastoderm, and thus be regarded as showing a tendency 

 toward a "peripheral gastrulation." Its appearance in an unin- 

 cubated chick blastoderm would favor this view (Fig. 65). On the 

 other hand, the fact that it first arises at the anterior margin and 

 is not a continuation of a dorsal lip (Fig. XI), would indicate that 

 it was not comparable to the margin of the selachian blastoderm. 

 The answer to this question, however, turns upon the view one takes 

 as to the extent of the blastopore. I cannot agree with those in- 



"In the series shown in Fig. 10 hut a single nucleus was found beneath the 

 overgrowth (Fig. 2.")), and this one had doubtless arisen from the nucleus 

 lying below the zone of junction when that region formerly occupied the 

 margin of the blastoderm. 



