1 8 LABORATORY OUTLINES FOR EMBRYOLOGY 



ranged in layers which 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 colum- 

 nar cells, and is called the ectoderm or epiblast. The one 

 between the ectoderm and the endoderm is composed 

 of irregular cells, and is called mesoderm or mesoblast. 

 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 fold- 

 ing. If this folding is toward the inner part of the body, 

 it is called an imagination] 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 uniform- 

 ity, 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 



