CHICK, 33 TO 38 hours' incubation 17 



third mesoblastic somite. The remainder of the neural 

 tube is the spinal cord or myelon. 



What is the number of the mesoblastic somites? What 

 change has taken place in the primitive streak? Locate 

 the notochord. 



The heart lies ventral to the hind-brain. WTiat shape 

 is it? What is the comparative size? The two broad 

 tubes which are connected with the posterior end of the 

 heart are the mtelline veins. A single tube is connected 

 vdth. the heart at the anterior end. This is the ventral 

 aorta. Remember that veins always carr}- blood toward 

 the heart and arteries carry blood away from the heart. 



Make a drawing of the thirty-six hour whole mount and 

 label all the parts. 



b. Transverse Sections 



Sections cut perpendicular to the long axis of the body 

 are transverse sections. It is ver>' difficult to cut sections 

 so that they are exactly perpendicular to a given plane. 

 Many of the sections labeled transverse sections are 

 tipped a little to one side or the other. The sections are 

 cut of uniform thickness and arranged in regular order. 

 Those on most of your slides are fifteen microns thick. A 

 micron is one-thousandth of a millimeter. By counting 

 the number of sections in a series the exact length of the 

 embryo may be calculated, pro\'ided no sections have 

 been lost. The position of any section may be ascer- 

 tained by determining its number in the series. In a 

 transverse section of a chick embryo there are found three 

 kinds of cells, and these different kinds of cells are ar- 



