20 THE HISTOGENESIS AND GROWTH OF THE OTIC CAPSULE AND ITS 



form. After considering these, we shall be prepared in the second part of this paper 

 to take up the alterations in the periotic reticular tissue that lead to the formation 

 of the periotic spaces. 



CONDENSATION OF THE PERIOTIC MESENCHYME. 



If one looks at the otic vesicle in a human embryo from 4 to 5 mm. long, just as 

 the endolymphatic appendage is becoming constricted off from the remainder of 

 the vesicle, it will be found that the mesodermal tissue surrounding it is about the 

 same in its appearance as that in other regions. There is the brain-wall, the otic 

 vesicle, the ganglion mass connecting them, a few blood-vessels, and the ectoderm; 

 otherwise there is to be seen only a more or less uniform mesenchymal syncytium 

 lying between these structures. Close against the vesicle the nuclei are perhaps 

 a little more numerous. This can be seen in figure 5, which is taken from an embryo 

 4 mm. long (Carnegie Collection, No. 588). The section passes through the otic 

 vesicle in its longest diameter and shows dorsally the endolymphatic appendage as 

 it appears at this time. Lateral to the otic vesicle is the primary head-vein. A 

 network of capillary vessels is spreading over the brain-wall, not extending quite 

 to the ventral median line. Along the median margin of this sheet of capillaries 

 there forms a larger channel which gradually separates itself from the capillaries 

 and takes part in the formation of the basilar artery, as has been described in the 

 chick and pig by Sabin (1917). The mesenchymatous tissue is denser in some 

 regions than in others. The nuclei are quite sparse ventral to the brain-wall near 

 the median line, becoming perceptibly more numerous as we approach the ear- 

 vesicle. This increase in the number of nuclei in the neighborhood of the vesicle 

 marks the beginning of the mesodermal condensation that is to form the otic 

 capsule. It is not yet possible, however, to outline a definite layer of these nuclei. 



When embryos are examined that are a little older than this it is found that 

 a condensation of the mesoderm around the otic vesicle can be clearly recognized. 

 8uch a stage is shown in figure 6, which is from a photograph of a section of a human 

 embryo 9 mm. long (Carnegie Collection, No. 721). Under low magnifications it 

 is apparent that the mesoderm in the region of the vesicle is denser than the adjoin- 

 ing mesoderm, and particularly so on the lateral and ventral surfaces of the vesicle. 

 The condensation of the mesoderm is also beginning on the median surface of the 

 vesicle, but the process there is somewhat slower. The endolymphatic appendage, 

 however, is free from any surrounding condensation; the mesoderm appears to be 

 unaffected by its presence. The section in figure 6 passes transverse to the long 

 axis of the vesicle. A small portion of the brain-wall is shown that is slightly 

 retracted from the surrounding mesenchyme. The area of condensed tissue sur- 

 rounding the vesicle is thick enough to extend from the surface of the vesicle to 

 about half the distance from the vesicle to the ectoderm. 



When analyzed under higher magnifications, it is found that the compact 

 appearance around the vesicle is due to several factors. As compared with the 

 mesenchymal syncytium of the adjoining parts, the nuclei here are slightly larger, 

 are more numerous, and are closer together. The intervening protoplasmic syn- 



