240 H. S. Harrison, 



nor the cavum perilymphaticum but follows the course of the perilym- 

 phatic aperture. No stage is passed through comparable with the 

 earlier one of Pelobates, as regards the course of the ductus reuniens. 



At 27 mm, the spatium sacculare and ductus perilymphaticus are 

 well developed. The foramen perilymphaticum superius is separated 

 fi^om the foramen peril, inferius by a slender bar of cartilage. The 

 latter aperture now opens into the fissura metotica. The ductus re- 

 uniens passes into the cavum cranii by a small aperture lying immedia- 

 tely anterior to the fissura metotica (cl figs, 27, 28). This is the 

 foramen j)erüymphaticum accessorium of Gaupp [7], and at a later stage 

 it becomes part of the metotic fissure by the disappearance of the nar- 

 row dividing band of cartilage. The question as to the significance 

 of this temporary foramen is of some interest. I believe it to have 

 been originally quite separate from the fissura metotica, A study of 

 figs. 18, 19, 20, 21 (of Pelobates), will aid in the comprehension of 

 this point. As I have said, the ductus reuniens, which in these figures 

 is seen to run in the auditory capsule, becomes included in the cavum 

 cranii through the disappearance of one bar of cartilage and the de- 

 velopment of another. A comparison of fig, 18 (of Pelobates) with 

 fig, 26 (of Rana) will make this clear. If however, the tract of car- 

 tilage which, in fig. 20, is seen to separate the ductus reuniens from 

 the cavum cranii, were to disappear, we should have a condition iden- 

 tical with that shown in fig. 27, in which the aperture leading into 

 the cavum cranii is the foramen perilymphaticum accessorium. The 

 latter aperture is in fact the necessary consequence of the secondary 

 inclusion of the ductus reuniens within the cavum cranii. By its 

 fusion with the fissura metotica it has aided in the forward extension 

 of this aperture, a process which has also probably been assisted by 

 the inhibitory action of the Saccus perilymphaticus on the process of 

 chondrification in this region. 



In a metamorphosed animal of 20 mm length, the indications of 

 the change in course of the ductus reuniens are very marked, espe- 

 cially in the light of the condition observed in Pelobates, It runs on 

 the medial side of the bar separating the foramen perilymphaticum in- 

 ferius from the for, peril, superius, and tlie bai- itself is strongly curved 



