126 EDWARD MCCRADY, JR. 



convex flexure in the lumbar region in stage 30. The heads, 

 gill arches, and hearts of the two specimens are strikingly 

 similar. The forelimbs differ principally in the degree of 

 flattening. And yet the stages could never be confused. For 

 in stage 30 the caudal end of the body has been flexed sharply 

 in the ventral direction until it is brought up under the heart. 

 The coiling is so extreme that the greatest linear dimension 

 of many specimens at this time is as little as 4.0 mm. This 

 length is less than that of any specimen since stage 23. 



Linear dimension is, of course, in most animals a notably 

 inaccurate clue to developmental stage, but this is perhaps 

 in an exceptional degree true of the opossum. A 6-mm. 

 opossum embryo may belong to the end of the ninth day 

 (stage 26), the end of the tenth day (stage 29), or the end of 

 the eleventh day (stage 31). And a 4-mm. opossum may be 

 just acquiring its first somites (stages 22 and 23), or it may 

 have the principal lobes of the lungs indicated (stage 30). 

 For this reason, though average measurements are occasion- 

 ally mentioned, they are given very little emphasis in this 

 paper. 



If it is true that the primary (concave) lumbar flexure was 

 due to the rapid development of the mesonephros, which made 

 the ventral portion of the trunk elongate more rapidly than the 

 dorsal portion, then it is probably true that the secondary 

 (convex) lumbar flexure is due to a spurt of growth in the 

 lumbar spinal cord which makes the dorsal portion of the 

 trunk elongate more rapidly than the ventral portion. This 

 suggestion has not been verified by precise quantitative 

 studies, but it is in harmony with the fact that the posterior 

 limb bud first becomes conspicuous at this time, and there is 

 a corresponding and easily visible development of the region 

 of the spinal cord corncerned with the prospective lumbo- 

 sacral plexus. In stage 29 there is a very early neural crest 

 formation in the lumbosacral region and no sign of motor 



Fig. 42 Reconstructions and sections of stage 30. The central reconstruc- 

 tion is from 17132. The sections and J and K are from 16158. I represents 

 only the levels of the sections, not the size of the specimen. 



