l8 LABORATORY OUTLINES FOR EMBRYOLOGY 



ranged in layers that are more or less definite. These 

 layers of cells are called germ layers. The germ layer 

 lying next to the yolk is composed of rather large, rounded 

 cells. This germ layer is the endoderm or hypoblast. 

 The one lying next to the shell and farthest away from 

 the yolk is composed of cuboidal and columnar cells, and 

 is called the ectoderm or epiblast. The one between 

 the ectoderm and the endoderm is composed of irreg- 

 ular cells, and is called mesoderm or mesohlast. All of the 

 organs of the body are formed from these three germ 

 layers. Most of these organs are differentiated from 

 regions of unequal growth with a consequent folding. 

 If this folding is toward the inner part of the body, it is 

 called an invagination; if it is outward from the body, it 

 is called an evagination. 



The section should be studied with the low power of 

 the microscope unless otherwise indicated. Make out- 

 line drawings with hard pencil and use different colors 

 to represent the germ layers. For the sake of uniformity, 

 color the ectoderm blue, the mesoderm red, and the 

 endoderm green. Do not draw the cells. Indicate on 

 your drawing of the whole mount the location of each 

 section. 



(i) Through the Fore -brain Showing the Optic Vesicles 



The head is free from the blastoderm. How has this 

 separation been brought about? Of what germ layers 

 is the blastoderm composed? The mesoderm is in two 

 layers, one lying next to the ectoderm and the other 

 lying next to the endoderm. The layer of ectoderm with 



