Genera of the E'oeth American Palaeozoic Bryozoa. 471 



but apparently forms a solid flat mass, composed of thin concen- 

 tric layers, a^ 5, <?, in direct contact with each other. In the fol- 

 lowing stage (Plate D, fig. 17) the external layer has expanded into 

 a round cell, which is separated from the internal mass, and forms at 

 this time a discoidal plate. The wall, at first formed of an uniform 

 transparent membrane, is now differentiated into three parts : the 

 external layer, d^ which completely invests the embryo ; an interior 

 layer, a^ which has preserved the essential structure of a^ fig 16, 

 and at this stage consists of a somewhat thick, obscurely cellular 

 layer, and is called the endocyst. Between the endocyst and the 

 peripheral zone, d^ there is already visible a calcareous deposit, 

 in the form of an opaque ring. The cavity of the body situated 

 between the endocyst and internal mass, before this, compressed 

 in a small compass, as shown in fig. 16, Plate D, by the enlargement 

 of the external layer into a round sac, becomes very spacious, and 

 the fatty globules, heretofore generally adhering to the external 

 layer, for the most part detach themselves and drop into the 

 spacious cavity. Finally, the internal mass which has preserved 

 its pyriform aspect, commences to show two small papillae, which 

 are the rudiments of the tentacles. Even at this early stage we 

 are able to see, with certainty, that the internal mass forms the 

 rudiments of the polyp, of which the inflated portion gives rise 

 to the tentacular crown, while the more slender portion is the 

 beginning of the alimentary canal. The differentiation of the 

 parts of the zooecium proceed in a very gradual manner and, as 

 illustrated in fig. 17, are still vague, but in the stage represented 

 by fig. 1 8, Plate D, the parts previously vaguely indicated are 

 seen distinctly separated. The rudiment of the polyp is clearly 

 seen, divided into the tentacular crown and the beginning of the 

 alimentary canal. Moreover, the fatty globules are definitely 

 separated from the outer envelope, and are irregularly dissemi- 

 nated in the cavity of the body. Another important change 

 which takes place -at this stage, is the appearance, a little in front 

 of the center, and just at the anterior part of the polyp, of a 

 round swelling of the endocyst. The disc first appears in the 

 form of a round swelling, sessile on the walls of the body, but is 

 soon elevated above the level by the circular uprising of the walls 

 of the endocyst, which is rapidly effected and gives rise to a 

 tubular conduit, terminated above by a disc, which is pierced by 



