334 H. D. REED 



sively in these relations, although increasing in size along with 

 the growth of the animal. The further development of the 

 fenestral plate is concerned with the growth and extension of the 

 original isthmus into all parts of the fenestral membrane. Fig- 

 ures 23, 24, 25, and 26 show the early mode of invasion. "\Mien 

 first formed, the dorsal growth of the isthmus is triangular, with 

 the apex reaching dorsally toward the stylus columellae, entad 

 of which it has a tendency to pass. In the 28-nDLm. larva this 

 cartilage has extended along the chondrified stylus, both caudad 

 and cephalad, until the triangle is l^eversed in position (fig. 24). 

 During this same period it has invaded the membrane entad 

 of the stylus, against which the latter lies, and in one specimen it 

 reached a level dorsad of that of the stylus. 



The plan of growth is still more clearly outlined in the 31-mm. 

 stage where the fenestral plate has spread for a considerable 

 distance into the membrane in the dorsal and caudal directions 

 (fig. 25). When the stylus joins the plate the cartilage of the 

 two, in the posterior extent of the stylus, comes in contact, and 

 fusion takes place, though both may be distinguished by the 

 size of the cells or by staining reactions or both. While new 

 cartilage is being formed all about the free edge of the plate, 

 growth is not uniform. There are two points, one above, the 

 other below the stylus, where growth is most active. Thi& 

 results in two chondrified rod-like structures extending caudad 

 into the membrane, as shown in figure 25. In this particular 

 specimen, these rods had a tendency to enclose an unchondrified 

 area of the fenestral membrane. In their caudal growth the 

 rods have nothing to do with the stylus or with the cells pro- 

 liferating from it. This condition is perhaps better illustrated 

 by transections. The growth from the original invading isthmus 

 itself is shown in figure 4 (d.g.) of a section taken at a level 

 marked s in figure 25. It also shows the independence of the 

 columella and fenestral membrane at this level. A section 

 through the caudal half of the fenestra (fig. 5), at a level marked 

 c.s. in figure 25, shows the curved lower bar of the developing 

 plate composed of cartilage cells within the fenestral membrane, 

 which contrasts with the location of the cells of the columella in 



