ii4 Vertebrate Embryology 



the first day, it may be well to give a very 

 brief summary of the whole process of de- 

 velopment. 



SUMMARY OF DEVELOPMENT 



A very careful study of the series of dia- 

 grams in Figs. 37 and 38 will greatly aid in the 

 comprehension of the more detailed descrip- 

 tion that is to follow, keeping in mind that the 

 dotted areas marked "op," in Fig. 37, are, in 

 reality, spaces and not tissue, as might naturally 

 be inferred from the diagrams. 



After having followed the development of 

 the various organs and systems of organs in 

 the frog, the only features in the development 

 of the chick that will be apt to give trouble are 

 the folding off of the embryo from the yolk 

 and the development of the amnion and al- 

 lantois, structures not found in the frog. 



An understanding of the way in which the 

 embryo becomes folded off from the rest of 

 the egg may, perhaps, be obtained in the fol- 

 lowing way : cut out four circles of cloth, say 

 75 cm. in diameter, of three different colors. 

 Put the two circles that are of the same color 

 together and then put these two circles between 

 the other two. 



