742 



articulation. It is prolonged over 49 sections, 15 ii thick. Imnie- 

 diatelj' ventral to the anterior portion of this |)lace of attachment 

 one sees in tlie sections 5, 6 and 7 (in antero-posterior sequence) a 

 connection tiiroiigii a small opening in the wail of the auditory 

 capsule, between the mesenchjm which in this stage tills the peri- 

 I3 mphatic space, and the mesenchyni outside the capsule. The posterior 

 margin of the opening is not clearly defined, so that it remains 

 dubious whether the hole is present in the next three sections or not. 

 On the contrary the margins of Ihe o|)ening in the 255 m.ni. long 

 embryo, are clearly defined. The attachment of the hyomandibular 

 to the capsule takes place here on about 59 sections 30 ft thick (in all 

 the other embryos the sections are 15 ,« thick). 



The opening reaches from the 8"' to the 25''' section (counted 

 antero-posteriorally). It is closed by a deeply red stained connective 

 tissue, which also helps to connect the hyomandibular to the skull, 

 and which is rather conspicuously surrounded by the blue colour of 

 the cranial cartilage. The opening lies in llie under pari of the 

 fossa for Ihe hyomandibular, wliich partly covers it. 



From the wax model of iVlr. P. J. ok Vries, made according to 

 tiife method of Born, one can see that the opening is not truly oval, 

 but i-ather kidney-shaped, because the under margin forms a re- 

 entering concavity. The meseuchym which formerly filled the peri- 

 lymphatic spaces, has to a large extent disappeared and been replaced 

 by a liquid, which is prevented from flowing out, by the connective 

 tissue closing the opening. 



The opening, owing to its position, has to be considered as the 

 homologue of the fenestra vestibuli, wiiich in Amphibians and Am- 

 niotes is closed by the stapes, and which according to general 

 opinion would be absent in fishes. 



Owing to the great length of the embryo, ii must have been 

 more or less fully developed, and it is im|u'obable that the fenestra 

 would not persist after birth. 



I, however, had no opportunity of examining adult material. 

 Irrespective of the autostylic Dipnoi and Holocephali, fishes are as 

 a rule hyostylic. Their powei'ful iiyomandibular functions in the 

 first instance as a suspensorium. This fact evidently has to do with 

 the absence of a fenestra vestibuli. Only two primitive forms viz. 

 Heptanchus and Hexanchus are amphislylic. Their hyomandibular, 

 owing to the firm attachment of the palatoqnadrate to (he skull, can 

 only feebly function as a suspensorium. It is therefore conceivable, 

 that the hyomandibular, at least in Heptanchus, may still have the 

 function of transferring vibrations to the auditory organ. 



