CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 21 



cytium is also denser and possesses wider trabeculae, with correspondingly smaller 

 spaces between them. This condensed tissue abuts, on the one hand, directly 

 against the epithelial wall of the vesicle and forms a limiting membrane, as can be 

 seen in places where the epithelium is retracted through shrinkage changes. On 

 the other hand, it is directly continuous with the general mesenchymal syncytium, 

 the transition between the two, however, being quite abrupt, as can be seen on 

 careful scrutiny. 



In embryos between 11 and 13 mm. long, which is just before the first semi- 

 circular duct is separated off from the main labyrinth by the apposition and 

 absorption of the intervening labyrinthine wall, the condensation of the mesoderm 

 has advanced in thickness and extent so that it forms a nearly complete capsule 

 for the epithelial labyrinth. Such a stage is shown in figure 7, which is taken from 

 a human embryo 11 mm. long (Carnegie Collection, No. 353). This capsule en- 

 casing the labyrinth is thicker and denser on the lateral and ventral surfaces of the 

 labyrinth, including the ventral pouch that is to form the cochlea. It remains 

 incomplete on the median surface in the region of the nerve terminations. This 

 latter space is occupied by the rootlets of the acoustic nerve-complex which bridge 

 the short distance between labyrinth and brain and which are invested by a rich 

 plexus of blood-vessels. It is this area that eventually becomes the internal audi- 

 tory meatus. Slightly more caudal, near the glossopharyngeal nerve, can also 

 be made out a deficient portion of the capsule that corresponds to the fenestra 

 cochleae (rotunda) and the aquaeductus cochleae. A third opening through the 

 capsule is brought about by the endolymphatic appendage. This does not become 

 encased by the capsule, but emerges dorsally to lie between the brain membranes 

 and the skull. At first this latter opening is one in common with the internal 

 auditory meatus. It very soon becomes separated off by the growth of the con- 

 densed tissue around the neck of the endolymphatic appendage. In figure 7 the 

 section passes through the long axis of the membranous labyrinth. Only the 

 vestibular portion is shown with the endolymphatic appendage opening out of 

 it. The section passes transverse to the thickened margins of the pouches that are 

 to form the superior and lateral semicircular ducts. 



Thus at this time there is completely formed a condensed area of embryonic 

 connective tissue surrounding the labyrinth that corresponds closely in form to 

 the cartilaginous capsule into which it is about to be converted. On examining it 

 under higher magnification there is found very little, aside from the condensation, 

 that distinguishes it as yet from ordinary embryonic connective tissue. The con- 

 densed appearance is due to several factors. In the first place, the nuclei are more 

 numerous in a given area. They also tend to be larger and rounder. Further- 

 more, the protoplasmic syncytium between the nuclei is denser, consisting of more 

 numerous and more branched trabeculae. In an embryo 16 mm. long, which had 

 been stained with iron hematoxylin and erythrosin (Carnegie Collection, No. 406) 

 the trabeculae between the nuclei appear granular. This appearance is due to the 

 presence of minute nodes that are found along the trabeculae and which are stained 

 deeply by the erythrosin, and add to the density of the tissue. Similar nodes are 



