70 



TEXT-BOOK OF EMBRYOLOGY. 



\ / Gastrulation. In the bird as in the 

 lower forms gastrulation is the process 

 by which the single layered blastula is 

 converted into the double layered gas- 

 trula. In the bird the blastula consists -, 

 of a disk of cells, the blastoderm, resting - 

 upon the yolk, the blastoccel being a 

 shallow cavity beneath the center of the 

 disk. Between the twentieth and tenth 

 hours before the egg is laid the cells of 

 the blastoderm are rearranged so that 

 a sector of the posterior or caudal third 

 becomes thinner and composed ^)f only 

 a single layer of cells (Fig. 41)^ In 

 front of the sector there is a graaual 

 increase in thickness and at the anterior 

 or cephalic border the blastoderm may 

 be seven cells thick. Around the caudal 

 edge of the sector the germ wall is inter- 

 rupted so that the margin of the disk 

 rests dir ec tly upon the yolk . The bias to- 

 coel during this time becomes larger. 



^ Gastrulation is initiated by the tuck- 

 ing or rolling under of the caudal margin 

 of the sector. The cells thus rolled in 

 or involuted continue to proliferate and 

 at the same time seem to migrate for- 

 ward toward the cephalic border and 

 outward toward the lateral borders of 

 the blastoderm (Fig. 42). They do not 

 at first form a complete layer but are 

 more or less scattered. This new layer 

 of cells is the entoderm while the original 

 layer which now lies over it comprises 

 the ectoderm. The two layers are con- 

 tinuous at the margin of the sector 

 where involution began. This margin 

 is the anterior lip of the bias to pore, a 

 minute cleft between the margin and 

 the yolk behind it, which leads from the 

 exterior into the space, now the arch- 

 enter on, beneath the double layered 



