1901.] NATURAL SCIENCES OF I'lIILADELPHIA. 247 



internal wall of this portion to indicate the proximal, connecting 

 part of the tubal cord. The proliferation has very likely opened 

 out with the formation of the " lateral recess," and has been merged 

 into the dorsal wall of the latter. 



It thus appears that in the present stage the Eustachian cord for 

 the greater part of its length has undergone remarkable fragmen- 

 tation, having broken up into a number of sections of variable 

 length. Each of these fragments, however, retains exactly the 

 same relations to the surrounding structures that the corresponding 

 part of the cord showed in the preceding stage. It is quite possible 

 that the various fragments may still be connected by the trans- 

 parent cell-walls of the cord, and in that case the apparent frag- 

 mentation is simply due to the restriction of the more vital, stain- 

 able portions to areas less subject to the action of unfavorable 

 forces. I am somewhat inclined to consider this the actual condi- 

 tion in the present stage, since in a longitudinal series I have been 

 enabled to follow out with great care a pale, almost transparent 

 cord connecting some of the fragments. Posteriorly this cord 

 approaches very closely the wall of the pharynx. I have not been 

 able to make out any distinct connection between the two, but 

 their proximity would incline me to believe such a union to exist. 

 Still I have not been able to satisfy myself on this point. 



I have not been able to determine to my satisfaction the factors 

 which have been concerned in the degeneration of the Eustachian 

 cord. One of them is probably to be found in the pressure exerted 

 by the surrounding structures, particularly by the two muscles, 

 already mentioned. Owing probably to its unfavorable position 

 the tubal cord appears to have little, if any, power of indepen- 

 dent growth. It therefore may have been acted on by the growth 

 anteriorly of the head whereby a pull has been exerted on it, 

 causing its wall to extend and its contents to be restricted to more 

 or less limited regions of the cord. 



Stage VII. — Tadpole of 21 mm. Hind limbs well developed. 



This stage very closely resembles the preceding. Owing to an 

 accident the more anterior sections of the Eustachian cord in the 

 specimen examined are lacking, but I have no doubt but that this 

 portion of the cord in the present stage corresponds in all essential 

 respects with that in the preceding, since in the succeeding stage 

 the anterior portion is very similar to that in stage VI. 80 far 



