FIXED AND STAINED EMBRYOS THE CHICK 15 



FIXED AND STAIND EMBRYOS THE CHICK 



i. Embryo with from Three to Six Mesoblastic Somites 

 (Twenty-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 embryo the blastoderm is 

 folded back under the embryo for a short distance. 

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

 the embryo from the yolk. On either side of the middle 

 of the embryo, 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 will form 

 the brain and the remainder will form the spinal cord. 

 Lying beneath this neural groove is a thickened 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 embryo? The plate of cells 

 extending tailward from the mesoblastic somites is 

 called the segmental plate. With the increase in develop- 



