DEVELOPMENT OF THE PHTLACTOL^MATOUS POLTZOA. 503 



The young statoblast now assumes an oval lenticular form, 

 while the cavity of the cystogenous layer disappears by the ap- 

 proximation of its walls ; and this layer now lies on the free 

 surface of the formative mass in the form of an oval plate com- 

 posed of two cellular layers, which pass into one another at their 

 margins. 



The cystogenous layer next extends itself at its margins, and 

 gradually grows round the formative mass nearly in the same 

 way that the fold of the amnion grows round the embryo. 



At the same time there appears, between the two cellular layers 

 of the cystogenous portion, a strongly refringent membrane. This 

 is the foundation of the chitinous envelope of the disk, and is 

 apparently a secretion from the cells of the outer layer. 



The formative mass has in the mean time become differentiated 

 into long fusiform cells, filled with strongly refringent granules 

 and without any apparent nucleus. 



The cystogenous layer now continues to grow round the disk, 

 depositing as it proceeds the chitinous secretion between its two 

 layers, and its margins have begun to approach one another at 

 the opposite side of the disk. The inner layer has, however, 

 become less distinct, a condition which is only a precursor of its 

 complete disappearance ; while the cells composing the outer 

 layer at that part where it bends round the_ sharp edge of the 

 disk have become very much elongated, and the outermost ones, 

 here gliding away from the disk, have arranged themselves in two 

 series, an upper corresponding to the upper side of the disk, and 

 a lower corresponding to its lower side. The cells of each of these 

 series impinge by their bases on the bases of those of the other 

 in a plane which corresponds to an extension of the sharp edge 

 of the disk. 



A remarkable phenomenon now shows itself in these elongated 

 cells. They begin to secrete chitin, not only from their bases as 

 they have all along done, like all the cells of the outer layer, but 

 from their sides, so that a chitinous secretion is thus deposited in 

 the intercellular spaces. 



The secretion from their bases forms a thin dou.ble lamella, 

 which runs round the margin of the disk in the form of an exten- 

 sion of its sharp edge ; while the secretion from the sides of the 

 cells forms, both on the upper and lower sides of this lamella and 

 on the adjacent parts of the disk, a series of short chitinous tubes, 



