Genera of the I^oeth American Palaeozoic Bryozoa. 467 



method ; that is, there is at first a sinking in of a part of the 

 blastula inside the other part, afterward the bringing together of 

 the margins of the depressions thus formed. Figures 1-7 represent 

 the first stage ; figs. 8, 9, 10, Plate C, the second. The sinking in 

 of one-half the blastula into the other part is not effected rapidly, 

 as frequently happens, reducing at once the cavity of the body 

 to a simple slit, but is effected, on the contrary, in a very slow- 

 manner. It is during this slow process of invagination that the 

 egg assumes an elongate form, as shown in fig. 7. It is toward 

 the end of the process of invagination that the elongate form is 

 most decided. Afterward, by degrees, the egg recovers its round 

 form. Pigure 9, which is an exterior view of the stage represented 

 in section by fig. 8, shows the form already less elongated. Figure 

 10 shows a complete return to the round form. Figures 8, 9 show 

 the archigastrula already well formed ; nevertheless it is not until 

 the following stage, fig. 10, that the form is completed. We 

 have not only the constriction of the opening of the invagi- 

 nation which forms the mouth of the gastrula and the constriction 

 of the cavity of the body to a simple slit, but there are other 

 changes of great importance. The surface begins to have a 

 general covering of fine vibratile cilia, and the embryo is swollen 

 a little forward of the middle, so as to present at this portion a 

 line (c) of the greatest width ; this line separating the primitive 

 uniform exodermic sac into two faces, the portion above the line 

 being the oral face, and the portion below, the aboral face. 

 The stage illustrated by fig. 11 shows another change equally 

 equally important ; the ciliary covering hitherto disseminated 

 over all the surface begins to be concentrated on the oral face, 

 the aboral face being without it. At the same time at the 

 point of greatest width the exoderm begins to separate from the 

 internal layer, which it has heretofore carpeted, in such a man- 

 ner as to give rise to a sort of fold, formed as a thick bourrelet. 

 This is developed more toward the aboral than toward the oral 

 face, making the two faces hitherto unequal of the same dimen- 

 sions. On account of the formation of this bourrelet the cavity 

 of the body, heretofore a simple slit, begins to be enlarged at 

 this point {cc) ; but at the same time that this local enlargement 

 appears, it begins to disappear at another portion. All that part 

 of the outer layer which formed the aboral face, in effect com- 



