54 BULLETIN OF THE 
the margin in Lepralia (Fig. 73) one finds a thick ectodermal layer, 
composed of columnar cells, but the mesoderm consists of an irregular 
thick mass of cells, some of which appear to be amoeboid. They how- 
ever show no signs of having been derived from the outer layer. The 
condition of the budding margin of Escharella resembles that of Lepralia. 
In older parts of the body wall, where the ectoderm is reduced to an ex- 
tremely thin layer, only scattered mesodermal cells appear, and these are 
amceboid or mesenchymatoid. 
On the other hand, one finds in the body wall, around the nascent 
neck of the polypide (Plate X. Fig. 88), even to a late stage, both ecto- 
derm and mesoderm well formed as layers. The ectoderm is a columnar 
epithelium ; the mesoderm is flatter, and often its cells are not sharply 
delimited from one another. It is thus perfectly evident, to my mind, 
that the mesoderm has in general lost its original epithelial character 
in the marine Bryozoa, although it has retained it in Phylactolamata. 
Whenever it does exist in the former group as an epithelium, it is at the 
budding regions (neck of polypide, and Figures 74, 75, 78, 79, ex.). 
Origin of the Polypide, — There are very few problems in modern 
morphology, I fancy, the history of whose investigation shows a less 
satisfactory aspect than that of the origin of the polypide in Gymnola- 
mata, It is hardly to be wondered, however, that investigators have 
sought for another interpretation of the process than the most obvious 
one, because that seemed to oppose many long cherished and wellnigh 
universally held dogmas. While the first recognition of the animal 
nature of marine Bryozoa, which we owe to the studies of Bernard de 
Jussieu in 1742 and John Ellis in 1755, brought with it a knowledge of 
their colonial nature, yet it was not until much later that the most 
characteristic part of this process — the formation of the polypide — 
was clearly observed. Grant (’27, p. 115) and Farre (’37, pp. 400, 409, 
415) first described the process by which is formed this complex of or- 
gans, and settled once for all the controversy which had sprung up as to 
whether these animals were truly stock-builders. Under the influence 
on the one hand of the endosare theory of Joliet (’77), and on the other 
hand of the view promulgated by Hatschek (’77), that similar organs 
in larva and polypide are equivalent as far as regards their origin from 
the germ layers, the more important papers ! between ’77 and ’90 main- 
tained either that the polypide arose independently of the body wall, 
1 Excepting those of Barrois, who, from the study of the favorable material 
presented by metamorphosing larv®, has persistently maintained the correct 
interpretation, 
