ORIGIN OF THE MESODERM 



463 



From the extra-embryonic portion the following are developed : 

 embryonic membranes and appendages, 

 extra-embryonic portions of the vascular system, 

 extra-embryonic coelom (exocoelom). 



Probably the most understandable method of making much of what 

 has been said clear, is to use Professor Reese's method of illustration : 



"An understanding of the way in which the embryo becomes folded 

 off from the rest of the egg, may perhaps be obtained in the following 

 way : Cut out four circles of cloth, say 75 cm. in diameter, of three dif- 

 ferent colors. Put the two circles that are of the same color together 

 and then put these two circles between the other two. 



"Let these superimposed circles represent a greatly enlarged blas- 

 toderm that has been removed from the yolk to which it was originally 



Fig. 270. 



Schematic diagrams showing the extra-embryonic membranes of the chick. 

 The egg is cut longitudinally while the embryo (which lies at almost right angles 

 to the egg), is cut transversely. A, embryo at about 48 hours ; B, same at about 72 

 hours; C, same at about five days; D, same at about fourteen days. (After 

 Duval. ) 



attached. The upper layer of cloth will represent the ectoblast, the bot- 

 tom layer will represent the entoblast, and the two similarly colored 

 layers in the middle will represent the two layers of the mesoblast after 

 their separation. 



"As the yolk takes no actual part in the formation of the embryo 

 other than as a supply of the food for the growth of the constantly en- 

 larging chick, it may be omitted from our model. 



"Now spread the cloth-blastoderm upon a table and place under 

 its centers a small object, such as a bottle. If now, the fingers of one 

 hand be pushed under one end of the bottle, carrying, of course, the three 



