28o The AUiirator and Its Allies 



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mesoblastic part of the amnion (a). The amnion 

 may be traced through about 130 of the 200 sec- 

 tions into which this embryo was cut. 



Figure 1 2e is nearly one fourth the length of the 

 embryo posterior to Figure I2d; it is approximately 

 in the middle region. The diameter of the embryo 

 has been gradually decreasing until now it is very 

 much less than in the head region. The section 

 being behind the head-fold the entoderm (en) is 

 nearly flat and the enteron is quite unenclosed. 

 The canal of the spinal cord (sc) is smaller in 

 proportion to the thickness of its walls, and the 

 notochord (;?/) is somewhat larger than in the 

 preceding sections. In proportion to its extent, 

 the ectoderm is very thick. Under the notochord 

 the dorsal aortas {ao) are seen as two large, round 

 openings in the mesoblast. On the left side the 

 section passes through the center of a somite and 

 shows a small, round myocoel (myc). The meso- 

 blastic layer of the amnion (so) is distinct through- 

 out from the ectoblastic layer (a). 



The most important structures to be here noted 

 are the first rudiments of the Wolffian ducts (wd). 

 They are seen in the present section as lateral ridges 

 of mesoblast projecting outward and upward 

 toward the ectoblast, which suddenly becomes 

 thin as it passes over them. These ridges or cords 

 of mesoblast are as yet quite solid. They arise 

 suddenly at about the eightieth section of the 

 series of two hundred and may be traced through 



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