106 EDWARD MCCRADY, JR. 



folded into a deep concavity. When the amniopore is actually 

 closed (during stage 29) the concave bending stops, and very 

 shortly thereafter (stage 30) the flexure reverses itself. I 

 mention this only as a simile, for it is difficult to imagine 

 a force which might correspond to the closing of the string 

 loop. 



In the human embryo of the early part of the fifth week 

 there is a similar concave curvature of the back. The amnion 

 in that case has formed very much earlier and not by a folding 

 process at all, which may be taken as evidence that there is 

 no causal relation between the origin of the amnion and the 

 formation of this flexure. To explain this case it has been 

 suggested that the 'weight' of the yolk sac, which 'hangs' 

 from this point, is responsible for pulling the back down. 

 I don't know whether the yolk sac has any weight or not in 

 the fluid in which it is immersed, also I don't know just what 

 'hangs' means when the site of implantation may be dorsal or 

 ventral with equal frequency and is often lateral. But the 

 final answer to the weight-of-the-yolk-sac theory comes from 

 the opossum in which the yolk sac is the lining of the entire 

 embryonic vesicle, and the embryo actually descends into and 

 becomes completely surrounded by its own yolk sac. None- 

 theless, the opossum embryo develops a good primary lumbar 

 flexure, which obviously in this case cannot be caused by any 

 weight of the yolk sac. 



More likely than a connection with either the process of 

 formation of the amnion or the weight of the yolk sac, is the 

 possibility that, like the other flexures, this one is due to local 

 differences in growth rates. If so, at the time of its forma- 

 tion growth in the thoracic and lumbar regions should be 

 more rapid on the ventral than on the dorsal side of the body. 

 There is good reason to think this is so, for the rapidly de- 

 veloping mesonephros is not only at exactly the right place, 

 but in both man and the opossum it is at exactly this time 

 developing tubules which are lengthening rapidly and being- 

 twisted into S-shapes for lack of space. This is also true of 

 the 55-hour chick, which corresponds to a stage 28 opossum. 



