40 BRYOZOA ECTOPROCTA. 



According to the more recent researches of Seeliger, Davenport, Kkaepelin, 

 Braem, and Oka on the development of the buds, there can be no doubt that 

 the inner layer of the bilaminar sac is to be derived by invagination from the 

 ectodermal layer of the parent, the outer layer, on the contrary, owing its origin 

 to the mesoderm of the latter. Some of the earlier observers had derived the 

 whole bud from the ectoderm of the parent (Nitsche, Ehlers, and Clapar1<;de), 

 while Haddon held that all the three germ-layers of the parent take part in the 

 formation of the bud. According to Joliet (No. 17), on the contrary, the first 

 rudiment of the polypide iu Eucratea is derived exclusively from the funicular 

 tissue (endosarc), i.e., from a layer which we regard as belonging to the 

 mesoderm. The first rudiment of the polypide is said here to consist of a mass of 

 similar cells, which become arranged only secondarily into two layers, the inner 

 cells acquiring an epithelial character and surrounding a central cavity, while 

 the superficial cells yield the outer layer. 



The first change which can be remarked in the bilaminar polypide- 

 sac consists in the formation of a diverticuhim (Fig. 20 A, d ; 

 23 B, d), which represents the first rudiment of the enteric canal, 

 especially that of the stomach and hind-gut. The origin of this 

 diverticulum can be traced to a constriction or infolding of the 

 wall of the primitive sac taking place from each side. In conse- 

 quence of this, the lumen of the enteric rudiment at first remains 

 connected along its whole length through a narrow slit Avith the 

 lumen of the rest of the sac. As the intestine becomes more 

 differentiated this slit becomes narrower, but never completely 

 closed, for it persists as tlie anal aperture (Fig. 20 C, a). In the 

 following stages a second diverticulum develops at the opposite 

 (oral) side of the polypide-sac (Fig. 20 B, oe) ; this diverticulum, 

 which represents the rudiment of the oesophagus, grows out towards 

 the blind end of the first diverticulum and fuses with it (Fig. 

 20 C). After communication has been established at the point 

 where the two diverticula come into contact, the intestinal canal 

 is essentially complete, the so-called stomach-caecum developing at 

 a later period. 



The description here given of the development of the enteric canal is founded 

 upon the statements of Barrois (No. 9), Prouho (No. 28), Braem (No. 45a), 

 Davenport (Nos. 11 and 46a), and Kraepelin (No. 50). A modification of 

 this process was described by Nitsche (No. 23) in Fluviatella and Flustra 

 ■niembranacea, and more recently by Ostroumoff (No. 26), the formation of the 

 intestine taking place through two ingrowths which extend into the interior of 

 the sac-like jjol^'pide-rudiment, from right and left, meeting and fusing in the 

 median plane. The apertures left in front of and behind this fusion become 

 the oral and anal ajicrtures. " Tliis process resembles what takes place if we 

 hold an indiarubber ball with a double wall in our two hands and ])ress it on 

 each side with a finger until the finger-tips are separated only by the four-fold 

 wall of the ball." The oesophagus in this case would not have an independent 

 origin. Seeliger observed a similar origin for the alimentary canal in the buds 



