606 



dorsal wall of which is found the unique structure first described by 

 Corning ('88), and called by him the "oval". The oval is a differen- 

 tiated area of the swim-bladder wall in which the inner layer becomes 

 thinner or entirely wanting, so that the flat epithelium of the lining 

 comes to lie immediately on the vascular (middle) layer which develops 

 an elaborate capillary network (rete mirabile). The structure of the 

 swim-bladder wall in this area is therefore essentially like that of the 

 wall of the posterior chamber in the preceding type, and like that of 

 the wall of the pneumatic duct in the eel. This differentiated area is 

 circular or oval in outline, and is bounded peripherally by a ridge 

 formed chiefly of smooth muscle fibers which lie immediately under 

 the epithelium; these are developed in the edge of the inner layer 

 (muscularis), where that layer becomes interrupted at the margin of 

 the oval. There are other species, however, in which the ridge forming 

 the boundary of the oval is developed to such an extent that it forms 

 a circular fold which covers more or less of the peripheral portion of 

 the epithelial surface of the oval (the oval with the peripheral fold 

 is shown in cross section in Fig. 6). An oval of this kind is probably 

 to be found in Lota vulgaris (Corning, '88) : "Das hintere Ende zieht 

 sich in einen zarten Sack aus, welcher zwischen die Wundernetze und 

 die fibröse Wand der Schwimmblase zu liegen kommt. Dieser Sack 

 hat eine Länge von etwa 8 mm und eine Breite von 3—4 mm. Seine 

 W T andung besteht ebenso wie diejenige des Ovals aus einer ganz feinen 

 Platten epithellage, unter welcher zahlreiche Züge glatter Muskelfasern 

 verlaufen." In the trigger-fish (Balistes carolinensis), and in the hake 

 (Urophycis tenuis), and probably in many other species, the peripheral 

 fold, apparently even in its expanded condition, covers in the border 

 of the oval around its whole circumference. 



The essential similarity between the oval and the posterior 

 chamber in the Opsanus type of swim-bladder is obvious. Histologi- 

 cally, the two structures do not appear to differ in any important 

 particular ; if the dividing partition of the Opsanus swim-bladder may 

 be considered as being approximated to its posterior wall, with the 

 consequent obliteration of the included cavity, the essential structural 

 conditions of the oval may thereby be produced. If this comparison 

 may be taken as suggestive of the real relationship existing between 

 the swim-bladders of the two types in question, the posterior chamber 

 and the oval are to be considered as homologous structures. Further 

 discussion of this matter will be taken up in the conclusion of this 

 paper. (Schluß folgt.) 



