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 

 amoeboid 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 Phylactolsemata. 

 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 Gymnolse- 

 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 1 742 and John Ellis in 1 755, 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 endosarc 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 larvae, has persistently maintained the correct 

 interpretation. 



