16 GEORGE L. STREETER 



the true cartilage is deep blue on account of the avidity with 

 which its matrix takes the stain, whereas the precartilage shows 

 only a nuclear stain and therefore is only faintly colored, as 

 compared with the sharply demarcated and almost opaque car- 

 tilage surrounding it. The negative of this picture is presented 

 in material where there has been an intense nuclear stain with 

 subsequent decolorization of the cartilaginous matrix. Such a 

 condition exists in figure 7 but is more marked in specimens 

 where the stain is more intense, such as the series No. 972 of the 

 Carnegie Collection. Under such circumstances the area of pre- 

 cartilage appears as a dark field in the midst of the faintly 

 stained true cartilage. Depending upon the management of the 

 technique it is thus possible in embryos about 30 mm. long to 

 display the future cartilaginous canals; that is, the precar- 

 tilaginous areas which approximately correspond to them, either 

 as dark fields in a light background or as light fields in a dark 

 background. 



In the second figure of the series, figure 8, the area repre- 

 senting the future cartilaginous canal, is appreciably larger. 

 Its perimeter, compared with that of the canal in figure 7, is in 

 the proportion of 152 to 115, which are measurements in milli- 

 meters made on photographs taken at 100 diameters. By com- 

 paring the two figures it will be seen that the increase in size is 

 obtained by the encroachment of the precartilaginous area upon 

 the surrounding cartilage. The amount of this encroachment 

 represents the amount of true cartilage which has reverted or de- 

 differentiated into precartilage. In a similar manner the retic- 

 ular zone surrounding the membranous duct has enlarged at the 

 expense of the precartilage. The reticular zone as shown in this 

 figure, taken from a human embryo 43 mm. long, forms a distinct 

 and characteristic eccentric vascular field, but it undergoes its 

 greatest expansion soon after this period. 



In the 50 mm. embryo, as can be seen in the third figure of 

 this series, figure 9, the area of the reticular zone is about the 

 same in size as the whole precartilage area in the 30 mm. 

 embryo of figure 7. On comparing these two figures it becomes 

 apparent that there is just as much, and even more, precartilage 



