234 MANUAL OF ANIMAL BIOLOGY 



unless the proper temperature (38° C.) is again supplied. The 

 embryo of the egg will remain dormant without injury to the 

 embryonic cells for several days and then start to develop again 

 when given the proper temperature. 



The division of the blastoderm in the Hen's egg after fertilization 

 results in the formation of a great number of irregularly shaped 

 cells which lie near the center of the blastoderm. These are the 

 primitive ectoderm cells, and they soon form a few layers of cells 

 which overlie a small cavity on the surface of the yolk, which is the 

 blastocoel. This constitutes the blastula stage of the Chick 

 embryo, and it consists of flattened layers of ectoderm cells above 

 the blastocoel rather than a sphere of cells enclosing this cavity 

 as in the Frog. (W. fs. 31, 174.) 



The ectoderm cells continue to divide, and very soon, in the 

 region which is to become the posterior end of the embryo, they 

 begin the formation of the gastrula with ectoderm and endo- 

 derm. The two layers extend anteriorly and laterally, and in a 

 short time cover most of the blastodermic area. A median thick- 

 ening appears in the blastoderm at about the eighteenth hour of 

 incubation, which is known as the primitive streak, and indi- 

 cates the longitudinal axis of the animal. The ectoderm and 

 endoderm are in contact along the primitive streak, and it is also 

 in this region that the mesoderm layer arises. The first mesoderm 

 cells are given off in the posterior part of the primitive streak, but 

 they soon spread laterally and anteriorly, forming a definite third 

 layer between the ectoderm and endoderm. 



At the anterior end of the primitive streak, the ectoderm cells 

 divide very rapidly, and a definite anterior thickening can soon be 

 observed in this region, known as the head process. It consists 

 at first of only the ectoderm and endoderm layers, but later the 

 mesoderm is also drawn into it. The region of the blastoderm in 

 which the head process forms, i.e., just anterior to the primitive 

 streak, contains the rudiments of the body of the future embryo. 

 Very shortly there is an anteroposterior thickening of the dorsal 

 ectoderm in this region to form the beginning of the central nerv- 

 ous system. This corresponds to the medullary plate stage in the 

 Frog, and similarly there is an elevation of the lateral edges of the 

 medullary plate and a fusion in the mid-line to form the neural 

 tube. Lying below it will be found the endodermal notochord. 

 The lateral mesoderm segments into the myotomes, which, as in 



