48 



BIOLOGY OF THE LABORATORY MOUSE 



Because of its relation to the U-shaped pericardial coelom, the heart is 

 itself a U-shaped structure at this stage* with the base of the U lying just 

 cephalad to the anterior intestinal portal (Fig. 30). As the intestinal portal 



/ 



/ 



^^■•9': 





Fig. 27. — Stereoscopic photograph (Xio) of rat embryo, age 10^:1 days. {From 



Long and Burl inflame.) 



105 .ujv 235 x20 i..id„v, 23a x20 



Fig. 28. — Stereoscopic photograph (X20) of rat embryo, age lo^:^ days. Same 



embryo as Fig. 27, more completely dissected. (From Long and Biirlingamc.) 



moves caudad due to the ''zipper action'' which causes the progressive fold- 

 ing together and fusion in the mid- ventral line of the entoderm which bounds 



* In many vertebrates the heart originates as two entirely distinct primordia which 

 later fuse. As has been clearly shown by Goss (16) and by Burlingame and Long (8), 

 this is not the case in the rat. Our observations indicate that the condition in the 

 mouse corresponds closely to that in the rat. 



