THE EMBRYOLOGY OF THE OPOSSUM 



125 



XI. THE ELEVENTH DAY. Stages 30 and 31 



The first half of the eleventh day 



Stage 30. External distinctive features. The secondary 

 lumbar flexure. Bronchiolar buds. The pulmonary arteries. 

 The liver cords. The septum transversum and the pleuro- 

 pericardial membrane. Advances in the nervous system. 

 Miscellaneous details. 



External distinctive features. Stage 30 is distinguished 

 from stage 29 by the secondary (convex) lumbar flexure, the 



Fig. 41 Photographs of external views of stage 30. A, ventral view of 16158 

 after removal of all extra-embryonic membranes except the allantois and the por- 

 tion of the yolk sac covering it. B, lateral view of 16160 with amnion still 

 in place. C, lateral view of 16160 with only the non-vascular chorion removed. 



flattening of the forelimb club into a paddle, and the pro- 

 trusion of the allantois well beyond the body (fig. 42, 1). Also 

 for the first time there is a well-formed tail, and in most 

 specimens the posterior neuropore has closed. 



It is distinguished from stage 31 by the presence of the 

 naso-oral groove and the absence of prominent digital ridges 

 on the anterior limb paddle. 



The secondary lumbar flexure. If the photographs of type 

 specimens of stages 29 and 30 reproduced in the third normal 

 stage plate be compared with each other, it will be seen that 

 by far the most conspicuous difference is that caused by a 



