460 R. M. PACE. 



arcliigastrula with an opening to the exterior: no such stage 

 occurs in Flustrella hispida. 



Vigelius (25) studied the early stages in the development 

 of Bugula^ and noted the presence of four large dorsal cells 

 within the segmentation cavity and the subsequent division 

 of these to form the endodermic mass. 



Harmer (11), in his paper on Alcyonidium, mentions the 

 presence of a blastopore as occurring in that form. 



Prouho, in a paper (20) on Flustrella hispida, makes 

 no mention of the early stages of this form. In a later paper 

 (21), however, he describes the formation of endoderm in 

 tlio Cyphonautes larva. He states that at the thirty-two-cell 

 stage the embryo is flattened along an axis perpendicular to 

 the plane of the first segmentation, the four oral cells being 

 larger than the other cells and especially rich in yolk; these 

 four large cells subsequently become enclosed by the rapid 

 growth of the ectoderm. In the case of Alcyonidium 

 albidum, Prouho records that the endodermic cells each 

 divide into two before becoming enclosed by the ectoderm, 

 and he defines the blastopore in this case as the point at 

 which the ectodermic cells close over the four large cells. 



Braem, in his account (5) of the embryology of Paludi- 

 cella Ehrenbergi, points out that segmentation in the larva 

 of this species is total and almost equal. At the eight-cell 

 stage the segmentation cavity is visible, and the four upper 

 cells are somewhat larger than the lower cells. The sixteen- 

 cell stage is similar to that which occurs in Flustrella 

 hispida. At the thirty-two-cell stage the embryo is spheri- 

 cal, and forms a typical blastula; the blastula has a large 

 segmentation cavity, and the cells of the vegetative pole are 

 larger than those of the animal pole and do not increase in 

 number as rapidly. The four central colls of the vegetative 

 pole then become surrounded by the ectoderm cells, and are 

 pressed into the segmentation cavity. After their enclosure, 

 these four cells segment to form other endoderm cells, which 

 multiply and give rise to a many-celled archcnteron opening 

 to the exterior. A cell layer containing muscle fibres lies 



