12G PROCEEUINGS OF THE AMERICAN ACADEMY 



rowly limited, on account of the thickening of the walls of the head 

 without any corresponding increase in the thickness of the floor of the 

 depression. For the same reason the ventral depression has hecorne 

 deeper. Tliis series of sections shows that an ingrowth of mesoderm 

 has occurred even in the thinnest region of the partition, which so 

 thickens it as to separate more than in the previous stao^e the inner 

 from the outer portion of the cleft. The ingrowth is greater between 

 the ventral depression and the inner portion of the cleft than it is be- 

 tween the dorsal depression and the latter. It reduces the inner rem- 

 nant of the cleft to a narrow cavity, which, from its shape and position, 

 we may call the inner edge of the original cleft ; this persists as the 

 tympanum and Eustachian tube, but it communicates with the pharynx 

 proper only at its ventral end. That communication becomes the true 

 Eustachian tube. At this stage it points directly from the pharynx to 

 the dorsal depression (a), and the partition which separates it from the 

 same is still very thin (Fig. 15). 



By the time the foetus has reached the length of 1 G mm. the dorsal 

 dejwession (Fig. IG) has become much shallower, since the bridge 

 which separated it from the inner remnant of the cleft has become very 

 thick. The ventral depression, on the other hand, has increased in 

 depth. By these changes the floor of the external part of tlie cleft is 

 placed in a line parallel to the internal jiortion of the cleft. The prim- 

 itive tympanum and Eustachian tube continue to point toward the 

 dorsal depression. 



This is the stage that has misled many prominent investigators. The 

 ventral depression, which has now become very pronounced, has been 

 mistaken for the dorsal one, which at this time is quite inconsjjicuous 

 on account of the upward growth of the tissue that forms its floor. 



The length of the inner portion of the cleft, as Reichert first pointed 

 out, has not suffered any perceptible increase ; but, with a confusion 

 of these external depressions, one might think there had been a dor- 

 sally and posteriorly directed evagination on the part of the blind end 

 of the inner portion of the cleft. The dorsal part of this primitive 

 tympanum is, however, somewhat enlarged. 



The whole region of the external depressions is somewhat deeper in 

 an 18 mm. embryo, and the ventral depression especially has become 

 deep and narrow (Fig. 17). No other important changes are notice- 

 able, except an increase in the obliijuity of the plane of the partition, 

 principally due to the deepening of the ventral depression. 



"Wlien the embryo has reached the length of 25 mm. the ventral de- 

 pression is much deeper (Fig. 18, h). It can now be called the exter- 



