56 G. S. WEST. 



cliamber only retains a communication with the exterior, in 

 the adult animal, by the small pore opening at the anterior 

 angle of the orbit. The second invagination, which gives 

 rise to the inner chamber, arises at the side of the first one, — 

 in fact, almost as a lateral dilatation of it. 



The layers of tissue intervening between the inner chamber 

 and the maxillary bone are of greater thickness in the embryo 

 than in the adult, but there are only two of them. The 

 lining cellular layer is very thin, but there is a considerable 

 development of the delicate layer of connective tissue imme- 

 diately outside the maxilla. This layer contains much 

 branched connective-tissue corpuscles, with large oval nuclei, 

 and close to the outer surface of the maxillary bone these 

 corpuscles are very numerous. 



It is not until a late stage in development that the cham- 

 bers of the organ become lined by a distinct cuticle, and 

 the crenulated cuticle does not make its appearance until the 

 animal is still further advanced in growth. 



The pit-membrane of the embryo is of much greater thick- 

 ness than that of the adult, and is not so transparent. In a 

 well-grown embryo it is about 54 fx in thickness, but this be- 

 comes reduced to less than half as the animal becomes adult. 

 It contains relatively fewer blood-vessels, and the reticula- 

 tion of capillaries, which forms such a conspicuous feature of 

 the structure in the adult, is by no means so marked. On 

 the outer side of the pit- membrane there are several layers 

 of flattened cells which rest upon a single layer of cubical 

 supporting cells, with large round nuclei. On the inner sur- 

 face of the membrane is a layer of somewhat short and very 

 broad cells with round nuclei similar to those of the outer 

 layer of supporting cells. Between these layers are large 

 numbers of non-medullated nerve-fibres and a few blood 

 capillaries. The nerve-fibres have prominent nuclei, and 

 mostly run close to the inner surface of the membrane. 

 They are continuous towards the outer surface of the mem- 

 brane with branched nerve-cells, the latter i-eceiving pro- 

 longations direct from the innermost of the flattened cells on 



