CONTAINED PERIOTIC TISSUE-SPACES IN THE -HUMAN EMBRYO. 33 



This eccentric position gives the canal the largest arc that is possible in the space in 

 which it lies. It marks the point of thrust of the elongating duct against the cart ilag- 

 inous chamber that confines it and it is in this direction that the cartilage must be 

 excavated to make room for the further growth of the duct. 



The spread of the reticulum into the surrounding precartilage is rather slow at 

 first. There is very little advance made in fetuses between 30 mm. and 43 mm. 

 long, as can be seen by comparing figures 11 to 14. In figure 14 the reticulum can 

 be recognized as a crescentic-shaped area on the central side (toward the left) and 

 partially surrounding the epithelial duct. In the figure it is about 0.8 cm. wide at 

 its widest point. The surrounding precartilage is also of about the same width, 

 but it is uniformly wide around the whole circumference of the cartilaginous canal. 

 In fetuses about 50 mm. long the dedifferentiation of precartilage into reticulum 

 makes more rapid progress. The change is quite abrupt at this time. Figures 14, 

 15, 16, and 17 form a series in which is shown the alteration from a small amount 

 of reticulum to an almost complete reticularization of the cartilaginous canal. 

 These changes are found in fetuses varying from 43 mm. to 52 mm. long. In com- 

 paring these figures one would expect that the membranous duct would be found 

 progressively larger in the series of photographs if they were correctly arranged in 

 the order of their age. But it should be remembered that the tissues show different 

 degrees of response to the fixing reagents. This is particularly so in respect to the 

 epithelial duct; in figures 14 and 17 it is distended, as can be seen by its thin wall, 

 while in 15 and 16 it is contracted. The order in which they are arranged cor- 

 responds to their relative age, as far as could be determined by the records of the 

 fetuses and general appearances of the sections. 



In figure 15 there is a zone of precartilage, about 0.8 cm. wide in the photograph, 

 which in reality is true cartilage that has been dedifferentiated into precartilage. 

 The reticulum extends from the inner border of this to the membranous duct. In 

 figure 16, which is a section through the posterior canal of a fetus 50 mm. long 

 (Carnegie Collection, No. 184), the dedifferentiation of precartilage into reticulum 

 has occurred faster than that of cartilage into precartilage. There is practically 

 none of the latter to be seen; the whole of the space between the margin on the 

 cartilage and membranous semicircular duct is filled in by reticulum. Along the 

 central margin of the duct there are still seen thick clusters of proliferating nuclei 

 which are associated in part with the development of the blood-vessels and in part 

 with the modification of the reticulum that takes place around the wall of the 

 membranous duct. 



It has been noted that precartilage is free of blood-vessels, whereas the retic- 

 ulum is vascularized from the very first. Part of the dedifferentiation of precarti- 

 lage into reticulum consists of the invasion of blood-vessels into the precartilage 

 region. In the early stages of the reticulum the larger vessels hug closely against 

 the precartilage and continue to do so as the latter recedes from the epithelial 

 duct, as can be seen in figures 11, 12, and 14. Later, with the abrupt dedifferentia- 

 tion of the remaining precartilage into reticulum, the larger vessels do not follow 

 the receding margin of the cartilaginous canal, but form vascular arches that are 



