FIXED AND STAINED EMBRYOS THE CHICK 15 



FIXED AND STAINED EMBRYOS— THE CHICK 



I. Embryo with from Three to Six Mesoblastic Somites 

 (Twfenty-two to Twenty-four Hours' Incubation) 



a. Whole Mount 



The darker, more deeply stained part is the embryo. 

 The blastoderm extends beyond the embryo. What is 

 the shape of the area pellucida? What is its compara- 

 tive size? What is the appearance of the area opaca? 

 Is the area vasculosa present? 



At the head end of the embr\'o the blastoderm is 

 folded back under the embr>'o for a short distance. 

 This fold is called the head-fold. It lifts the head of 

 the embry^o from the yolk. On either side of the middle 

 of the embiyo, extending longitudinally, are two thick- 

 ened plates. In the head region they come almost to- 

 gether, but they diverge or spread out nearly flat toward 

 the tail. These thickened plates are called the neural 

 plates, and the groove between them is called the neural 

 groove. The anterior end of these neural plates wilf form 

 the brain and the remainder will form the spinal 

 card. L^-ing beneath this neural groove is a thick- 

 ened cord of cells, called the notochord. Does the 

 notochord extend the entire length of the neural groove? 

 Beneath the neural plates, or a little to either side of 

 them, are from three to six pairs of more or less cubical 

 thickenings. These are called the mesoblastic somites. 

 How many mesoblastic somites in your embr>'o? The 

 plate of cells extending tailward from the mesoblastic 

 somites is called the segmental plate. With the increase 



