Genera of the North American Palaeozoic Bryozoa. 475 



are plainly to be seen to be composed of two masses in close 

 apposition to each other [fig. 30, a]. We next find that the two 



Fio. 30. Showing the development of a statoblast (after Allman) . 



masses have lost their distinctness and fused together, and the 

 whole contents now appear to be composed of minute cells, con- 

 fined by common, external, transparent membrane, which is itself 

 plainly cellular [b]. The cellular condition of its contents must 

 not be confounded with true segmentation. The whole body 

 now begins to assume a more lenticular form, and within the 

 external envelope two other investments begin to show them- 

 selves. One of these, the more internal, extends over the whole 

 of the cellular mass, but the other is confined to the margin 

 of the lenticular mass, which it embraces in the form of a 

 ring [c]. No manifest structure, beyond a simple granular one, 

 can as yet be detected in these last formed envelopes; but the 

 ring is soon seen to be composed of distinct cells [d], which pre- 

 sent a bright central nucleus-like point, and a number of concen- 

 tric layers, which remind us of the secondary deposits in certain 

 vegetable cells. Up to this point the investments are all color- 

 less and nearly transparent, but we now find that the internal 

 envelope and annulus become more and more opaque, while the 

 former assumes a deep brown color and the latter becomes 

 yellow. They have both acquired a horny consistence, and the 

 annulus is composed of large hexagonal cells filled with air. If 

 now the whole be crushed under the microscope multitudes of 

 cells will escape all filled with minute, strongly-refracting cor- 

 puscles, but any further observation of the progressive develop- 

 ment of the contents, up to the opening of the statoblast and 

 the escape of the young polyzoan, is henceforth, on account of 

 the opacity of the covering, impossible. The statoblast having 



