546 Dr. C. A. Kofoid on Platydorina. 



posterior quartet of cells that they do in the 16-cell colony, 

 but are, as a rule, much larger, the postero-lateral pair 

 measuring from 20 to 30 yu in length, while the median one 

 reaches only a length of 15 to 18 /*. The five tails do not 

 lie in one plane, but share in the spiral of the colony, at 

 times, indeed, exceeding it in the degree of the twisting. 

 These structures are all subject to considerable variations and 

 irregularities of development (fig. 5), such as suppression, 

 inequality of members of pairs, differences in size and relative 

 development, in attenuation, and in degree of divergence. 

 These irregularities are often correlated with the loss of cells 

 in the colony due to parasites or other causes. The tails 

 nevertheless exhibit such a constancy of position and so much 

 of symmetry and regularity of development that they cannot 

 for a moment be considered as ephemeral features of little 

 structural importance. In their position they recall the pro- 

 tuberances noted by me (1898) at the posterior end of Pleo- 

 dorina illinoisensis. In Pleodorina, however, these struc- 

 tures are apparent only in disintegrating maternal colonies, 

 and it may be that they also indicate the point at which the 

 embryonic cup closes. On the other hand, in Platydorina 

 caudata these tails are present upon the colonies at the time 

 ot their escape from the maternal matrix, and persist through- 

 out the life of the adult, being permanent structures, charac- 

 teristic of the species. 



"YY ithin the outer sheath is a homogeneous gelatinous matrix 

 (fig. 1, m), which in Delafield's hematoxylin stains less 

 readily than the sheath. In the living colony no differentia- 

 tion of this matrix is to be seen, but, after staining, a delicate 

 sheath showing deeper colour is demonstrated about each of 

 the cells. In most places a considerable space intervenes 

 between this secondary sheath (fig. 4, s.sh.) and the inclosed 

 cell, so that the sheaths crowd upon each other and appear 

 to divide the field of the matrix into irregular polygonal areas. 

 These areas, as a rule, fill the greater part of the plate, leaving 

 unoccupied only a few corners, principally about the second 

 transverse row. The two poles of this swollen secondary 

 envelope are not of equal size, the inner being somewhat the 

 smaller, and slightly overlapped by those of the contiguous 

 cells. This is due to the intercalation of the cells of the two 

 sides of the plate and to the fact that the outer ends of the 

 cells are slightly nearer the surface of the plate than are the 

 inner ones. The gelatinous substance within the secondary 

 sheaths does not differ in structure or stainibility from that 

 of the surrounding matrix. As a result of the form of the 



