DEVELOPMENT OF FLUSTEELLA HISPIDA. 459 



especial interest to note that this tissue is from the first 

 distinct from the aboral ectoderm ; the difference in the 

 relative sizes of the oral and aboral ectoderm cells, which is 

 so marked in early stages, is noticeable throughout larval 

 life. 



Comparison with other Bryozoa. — As yet, compara- 

 tively little has beeu written on the early development of the 

 Bryozoa ; the most important papers on the subject being 

 those by Barrois, Joliet, Repiachoff, Vigelius, Harmer, Braem, 

 Prouho, and Calvet. 



Barrois has published descriptions of the early stages of 

 several Bryozoan larva3, and among others he dealt with (2) 

 that of Flustrella hispida. His observations, however, 

 were made solely upon entire eggs and larvnB. His descrip- 

 tions and figures are in entire agreement up to the thirty-two- 

 cell stage with the general results described in the present 

 paper, but he does not make any special mention of the 

 lineage of these cells, and he was also unable to study the 

 formation of the endoderm. As has already been pointed 

 out, Bai'rois erroneously describes a later stage, in which 

 both dorsal and ventral ectoderm are said to be composed of 

 small equisized cells, the two series being separated by an 

 equatorial ring of large cells (2, pi. xii, fig. 6), and he figures 

 this ring as being present in all subsequent stages. Had 

 Barrois sectioned any of his material, instead of relying solely 

 on external appearances, he would have seen that no equatorial 

 ring of single cells, such as he described, is present at any of 

 the stages figured. 



In another paper (3) Barrois describes the enclosure, in 

 S c h i z o p r e 1 1 a u n i c o r n i s , of four primitive endoderm cells 

 by epiboly. He also describes the formation of two bands of 

 mesoderm, which at a later period fuse with the endoderm 

 to form a single mesendodermic mass. 



Repiachoff (23), in a paper on Tendra zostericola, also 

 describes the endoderm as originating by the enclosure and 

 division of four large oral cells. He, however, states that 

 the process is followed in Tendra by the formation of an 



