104 Vertebrate Embryology 



blastoderm will exhibit two areas : around 

 the periphery of the disc will be seen a band 

 that is whiter and more opaque than the cen- 

 tral part; this is known as the area opaca; 

 the central, more transparent part is known as 

 the area pellucida. In the centre of the area 

 pellucida may be seen a white area (some- 

 times called the nucleus of Pander) which is 



Fba 



FIG. 36' LONGITUDINAL SECTION OF THE BLASTODERM AFTER 



THE COMPLETION OF SEGMENTATION. (After Duval.) 



ex, ectoderm. *.!, primitive entoderm. bbp, cells that form the 

 thickened rim of the blastoderm, c g^ sub-germinal cavity. 



the top of the flask-shaped mass of white 

 yolk which lies in the centre of the egg. 



If vertical sections of the blastoderm be 

 made, and examined under the microscope 

 (Fig. 36), it will be found to be composed 

 of two layers of cells ; the upper layer or 

 ectoderm (ex) composed of short columnar 

 or cubical cells of uniform size and closely 

 packed together, is more or less distinct and 

 sharply denned ; the lower layer of cells (in. i) 

 is much less sharply defined and is composed 



