CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 23 



In embryos about 16 and 17 mm. long the optic capsule takes on a definite pre- 

 cartilaginous character. This stage is shown in figure 9, which is from a photograph 

 of an embryo between 17 and 18 mm. long (Carnegie Collection, No. 144). The 

 embryo is listed in the collection as 14 mm. long, which is its measurement on the 

 slide. Instead of this we use here its estimated formalin measurement, so as to 

 conform to the other embryos, whose measurements are all given as in formalin. 

 The section passes sagittally through the labyrinth. Above is shown the posterior 

 semicircular duct and just below the center is shown the caudal end of the lateral 

 semicircular duct, at the point where it widens out to join the utricle. By this 

 time the differentiation of the tissue has advanced far enough so that one can prop- 

 erly speak of an otic capsule that is readily distinguished from any other condensed 

 connective tissue. The outlines of the capsule are everywhere distinct. It fuses 

 in part with the cartilaginous skull and it is continuous with the stapes. Embedded 

 in it is the epithelial labyrinth together with its ganglionated nerves. The capsule 

 envelops them entirely, except at the nerve entrance which is to form the internal 

 auditory meatus, also at a point in the region of the jugular fossa that is to become 

 the fenestra cochleae and at the opening through which the endolymphatic appen- 

 dage emerges. 



On comparing figure 9 with figure 8 it will be seen that in addition to an actual 

 increase in size the otic capsule is less uniform in appearance at this older stage. 

 There are areas of denser tissue, or, rather, areas of more deeply staining tissue, 

 which extend as streaks through the capsule inclosing other areas of less deeply 

 staining tissue. The areas of less deeply staining tissue are in the immediate neigh- 

 borhood of the semicircular ducts, completely encircling them and abutting directly 

 against the epithelial wall of the ducts, as in the previous stage. 



On examination under high magnification we find that the tissue forming the 

 otic capsule at this time (embryos 17 mm. long) has for the greater part been trans- 

 formed into precartilage. Precartilage, as seen in fixed material that has been 

 sectioned and stained by the usual methods, differs from condensed mesenchyme 

 chiefly in the alteration in the network of branching processes that extend between 

 the nuclei. In condensed mesenchyme these appear as a syncytium of delicate 

 refractile processes. In precartilage some of these become more sharply marked 

 and linear, and are looped together so as to inclose an irregular space near each 

 nucleus; the others become very finely subdivided and eventually disappear. While 

 these latter processes are disappearing the area in which they lie takes on a homo- 

 geneous appearance. It does not take the stain, but it is more opaque than the 

 inclosed spaces around the nuclei. Thus, instead of a syncytium the precartilage 

 tissue gives the appearance of cell-islands separated from each other by a homo- 

 geneous matrix. 



Regarding the exact structure of this slightly opaque substance our material 

 does not suffice to warrant an opinion. This question must be approached by 

 special methods. I may add, however, that remnants of fibrillar processes are 

 found embedded in this substance for some little time after the walling-off of the 

 encapsulated spaces or cell-islands. Each cell-island consists of a nucleus encap- 



