THIRD PERIOD OF DEVELOPMENT. 81 



which four or five mesoblastic somites have been 

 formed. This we will do through a consideration of 

 the whole embryo, and afterwards by looking at 

 them in sections. 



In Fig. 46 we see from the side an embryo formed 

 with five mesoblastic somites. These appear on 

 a side view already completely defined. From 

 front to back they diminish in size and formation r 

 the cavity of the mesoblastic somite especially being- 

 much smaller in the posterior mesoblastic somite. 

 Behind the region of the mesoblastic somites lies, 

 the undivided mesoblast fold. This, as compared 

 with the mesoblastic somites, does not appear sharply 

 defined laterally. It passes there straight over into* 

 the ventral hypoblast. It is only with regard to- 

 the further course of the development that it can 

 be stated how far the mesoblast folds stretch back- 

 wards. They reach right away over the gastrula- 

 mouth, and end with two large cells which form the 

 hinder boundary of the latter, and which we de- 

 scribed as the posterior polar mesoblast cells. 



In viewing the embryo from the back (Fig. 47), 

 one can, by different positions of the microscope, 

 get an insight into the condition of the mesoblastic 



G 



