l6 LABORATORY OUTLINES FOR EMBRYOLOGY^ 



in development of the embryo more mesoblastic somites 

 will be formed from the segmental plates. The posterior 

 region of the embryo, where the neural plates, the seg- 

 mental plates, and the notochord are indistinguishable, 

 is the primitive streak. Are there any blood vessels? 



Make a drawing of the twenty-four-hour whole mount, 

 showing the things described. Label the parts. Make 

 your drawing large enough so that it will almost fill the page. 



2. Embryo with from Ten to Sixteen Mesoblastic Somites 

 (Thirty-three to Thirty-eight Hours' Incubation) 



a. Whole Mount 



What is the shape of the area pellucida? What 

 changes have taken place in the area opaca? The 

 irregular, darkly staining parts of the area opaca are 

 blood-islands. This is the beginning of the formation of 

 the vascular area. How does the head-fold differ from 

 the head-fold in the twenty-four-hour chick. The neural 

 folds have united in the median line of the body and 

 formed a tube, the neural tube. How far back is the 

 neural tube closed? The anterior end of the neural tube 

 is considerably enlarged. This enlarged part is the fore- 

 brain. The parts of the fore-brain that project out on 

 either side are the optic vesicles. Posterior to the fore- 

 brain is a second enlargement of the neural tube. It is 

 not so large as the fore-brain. This is the mid-brain. 

 Posterior to the mid-brain are a few enlargements of 

 the neural tube, which constitute the hind-brain. The 

 hind-brain extends to and includes the region of the 



