CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 41 



along the inner border of the lateral duct. Along the other two ducts of the same 

 specimen (130 mm. crown-rump length) the reticulum has commenced the process 

 of space-formation, but complete channels are not yet established. A typical 

 section through one of the semicircular ducts in a fetus of this size, and this is the 

 oldest fetus studied, is shown in figure 23. As compared with the scalae in the 

 same fetus, as shown in figure 20, the space-formation along the ducts is very much 

 retarded. 



DEVELOPMENT OF SCALA TYMPANI AND SCALA VESTIBULI. 



The scala vestibuli may be regarded as an extension of the cistern downward 

 into the region of the cochlea and as such its growth starts from a focus opposite 

 the fenestra vestibuli (ovalis). The scala tympani in a similar way makes its first 

 appearance opposite the fenestra cochleae. From these two foci the scalae extend 

 gradually downward along the cochlear duct as two separate spaces which do not 

 communicate with each other until they reach the tip of the duct, where there is 

 finally developed a free opening between them known as the helicotrema. 



In their formation they go through a series of histogenetic changes essentially 

 in the same manner that has been followed in the case of the formation of the cis- 

 tern; this (as we shall see) consists of the enlargement of the spaces of theperiotic 

 reticulum that originally occupied this region, the enlargement being a result of 

 the disappearance of the protoplasmic bridges of the reticulum, whereby adjacent 

 spaces unite in the formation of composite larger spaces. This process continues 

 until there is a single continuous space extending down along the cochlear duct 

 representing each scala and at the margins of each of them there is developed a 

 membranous arrangement of the reticular cells which completely walls off the space 

 from the surrounding tissue. In these alterations in the reticular mesh and in the 

 formation of the surrounding membrane there is an active change in the form of 

 the reticular cells, which repeatedly adapt themselves to the new conditions. There 

 is no evidence to indicate that any other cells take part in the formation of the scalae. 



The first evidence of the formation of scalae is found in fetuses about 40 mm. 

 long, which stage is a little later than the first appearance of the cistern. In a fetus 

 43 mm. crown-rump length (Cargnegie Collection, No. 886), along the proximal 

 part of the cochlear duct on its basal surface there is a distinct widening of the 

 meshes of the periotic reticulum. This is the beginning of the scala tympani. 

 On the opposite side of the cochlear duct, where one would look for the scala vesti- 

 buli, the periotic reticulum retains its primitive appearance characterized by a 

 narrow and rather uniform mesh. Thus the scala tympani makes its appearance 

 slightly in advance of the scala Vestibuli that is, if we regard the latter as distinct 

 from the cistern. 



In fetuses 50 mm. long both the scala tympani and the scala vestibuli can be 

 plainly identified, although they are still very incomplete. A wax-plate recon- 

 struction of them, representing their form and their relation to the membranous 

 labyrinth in a human fetus 50 mm. crown-rump length (Carnegie Collection, No. 

 84), is shown in figures 26 and 27, being a median and a lateral view respectively. 



