THE ORGANS OF THE OUTER GERM-LAYER. 499 



reference to the epithelial parts. In the present instance there are 

 produced structures which are comparable with those existing in the 

 former cases, as has already been pointed out by Ko" LLIKER, SCHWALBE, 

 and others. 



The comparison may be carried into details. The parts arising 

 from the primitive auditory vesicle are at first surrounded by a soft, 

 vascular connective-tissue layer, as the neural tube and the epithelial 

 optic cup are. To the pia mater of the brain corresponds the 

 vascular membrane of the eye and the soft ear-capsule, or the 

 connective-tissue wall of the membranous labyrinth. Around all 

 three organs a firm envelope has been developed for the purpose of 

 protection ; around the brain the dura mater with the cranial 

 capsule, around the eye the sclerotica, and around the organ of 

 hearing the bony labyrinth with its periosteum. To these is to be 

 added still a third noteworthy agreement. In all three cases the 

 soft and firm envelopes are separated by more or less considerable 

 fissure-like spaces, which belong to the lymphatic system. Around 

 the neural tube the subdural and the subarachnoid spaces are found, 

 around the eye the perichoroid fissure, around the organ of hearing 

 the perilymphatic spaces, which have received in the cochlea the 

 special names of scalse (fig. 283 ST and SV}. 



The details of the formation of the enveloping structures around 

 the epithelial auditory vesicle are as follows : 



Soon after the auditory sac is constricted off from the epidermis it 

 is enveloped on all sides by a richly cellular mesenchyme, the indivi- 

 dual cells of which lie in an extremely scanty, soft, and homogeneous 

 intercellular substance, and possess each a large nucleus with a thin 

 protoplasmic covering having short processes. Gradually the envelope 

 is differentiated into two layers (figs. 279, 281). In the vicinity of 

 the epithelial canals the soft intercellular substance increases in 

 amount ; the cells become either stellate or spindle-shaped, in the 

 former case sending out long processes in various directions. There 

 is formed here that modification of connective substance known as 

 mucous or gelatinous tissue (figs. 281 and 283 </), in which there are 

 also blood-vessels. Outside of this the cells remain smaller and more 

 closely crowded together, and are separated from one another by thin 

 partitions of a firm intermediate substance. With an increase of 

 the latter the tissue soon acquires the character of embryonic 

 cartilage (&&). 



The further changes must be followed separately in the semi- 

 circular canals, the utriculus and sacculus and the ductus cochlearis, 



