ON THE SENSORY PIT OP THE CBOTALINJ). 55 



derived from several divisions of the maxillary branch 

 (cf. PI. 4, figs. 13 aud 14,, V.o. and V.m.). 



The pit-membrane is richly supplied with blood-vessels, 

 the main trunks of which enter at the periphery and run in 

 the folds alongside and internal to the nerve-trunks. From 

 these main vessels a capillary network extends through the 

 whole membrane, lying embedded in a sheet of fibrous con- 

 nective tissue internal to the nerve terminations. The blood 

 supply is derived from the ophthalmic artery. 



In a section of the pit- membrane, such as that figured 

 (PI. 4, fig. 15), the nerve terminations {n. t.) are seen imme- 

 diately beneath the outer cuticle (c), then within these 

 the numerous nerve-fibres {n.f.), then a few blood-vessels 

 (6. V.) embedded in the fibrous connective tissue, and finally 

 the inner cuticle. 



Most of the structure described was made out from serial 

 sections, cut through the anterior region of the head. In 

 the case of adult animals a mixture of equal parts of 1 per 

 cent, chromic acid and 1 per cent, hydrochloric acid was used 

 for decalcification ; but in the case of the embryos the follow- 

 ing solution, recommended to me by Mr. J. Bles, was used 

 with success : — 900 parts of 90 per cent, alcohol, 70 parts of 

 formalin, and 30 parts of acetic acid. For thorough decalci- 

 fication this mixture was required to be renewed every few 

 days for two or three weeks. 



Sections were also cut of the entire soft tissues removed 

 from the pit, and treated by Weigert's method for nerve 

 tissue. A figure is given of the edge of the pit treated in 

 this way (PI. 4, fig. 17) ; in it only the larger nerve-fibres 

 can be distinctly seen. 



II. The Structure of the Organ in the Embryo. 



The outer and inner chambers of the pit arise by invagina- 

 tions of the epidermis. That which gives rise to the outer 

 chamber retains a widely open communication with the ex- 

 terior, but that which ultimately gives rise to the inner 



