Development of the First Day 121 



It will be remembered that the blastoderm, 

 at the time of laying, is a small, disc-shaped 

 structure, like an inverted watch-glass, lying 

 on top of the egg just beneath the vitelline 

 membrane. 



In sections it shows two more or less dis- 

 tinct layers, the ectoblast, consisting of a single 



Fig. 39. — Part of a section through 

 the blastoderm after the formation of 

 the definite entoderm or hypoblast. 

 (After Duval.) 



f J-, ectoderm. ?"k, entoderm. ;«j, mesoderm. 



layer of closely packed, columnar cells : and 

 the lower layer cells, which are more irregular 

 in outline and are not arranged in so defin- 

 ite a layer as are the cells of the ectoblast 



(Fig. 36). 



One of the first changes that take place, 

 during the first part of the first day, is the for- 

 mation of a definite entoblast by the flattening 



