THE EMBRYOLOGY OF THE OPOSSUM 105 



head fold of the amnion has covered the limb buds. It is 

 distinguished from stage 29 by the presence of an amniopore 

 (fig. 35, D), and the absence of a deep naso-oral groove 

 (fig. 35, C). 



Though no distinct naso-oral groove has formed in stage 28 

 the olfactory placode is well developed and noticeably de- 

 pressed. The na so-lachrymal groove, which was quite con- 

 spicuous in stage 27, is becoming less so as the maxillary 

 process fuses with the lateral part of the naso-f rontal process. 



The forelimb rudiment has grown out until its length is 

 greater than the width of its base. It is now described as a 

 limb bud instead of a limb ridge. En route to the area vascu- 

 losa the omphalomesenteric vein sweeps around the anterior 

 or radial border of the limb bud. 



Though not confined to stage 28, perhaps the most con- 

 spicuous external feature which has not heretofore been de- 

 scribed is the primary lumbar flexure. 



The primary lumbar flexure. When the embryonic body 

 was relatively flat (early stages up to 26) its dorsal curva- 

 ture conformed to that of the rest of the vesicle that is, it 

 was slightly convex. During stages 26 and 27 the anterior 

 third of the body bent sharply in the ventral direction at two 

 points (cephalic and cervical flexures) and thus thrust itself 

 down into the yolk sac. During stage 27 the dorsal line from 

 the tenth somite back is fairly straight, but there is often a 

 slight concave flexure in the posterior third of the body. This 

 is the beginning of the primary lumbar flexure. 



During stage 28 this flexure becomes very marked. It 

 extends from about the tenth to the twenty-sixth somite and 

 is deepest at the level of the twentieth to the twenty-second. 

 It occurs at the time of constriction of the amniopore. The 

 amnion at this time may be compared to an open bag with a 

 loop of string tied around the mouth, which loop is gradually 

 being tightened so that the opening (the amniopore) is being 

 closed and the bottom of the bag (which includes the flat 

 thoracic and lumbar parts of the embryonic body) is being 



