I04 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 



Fig. 36- — Longitudinal section of the blastoderm after 



THE COMPLETION OF SEGMENTATION. (After Duval.) 



~ex^ ectoderm. /«.l, 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 ^^^. 



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 defined ; the lower layer of cells (in. i) 

 is much less sharply defined and is composed 



