OBSERVATIONS ON FRESH-WATER POLYZOA. 139 



the inner layer of the body-wall on its inner side, at the position in 

 which the buds are constantly developing. When a bnd is to be 

 produced, a portion of this sac is constricted off and gives rise to the 

 inner layer of the alimentary canal, while nil other parts of the young 

 polypide are formed by an invagination of the body wall. Thus, 

 the sac is being constantly constricted off, as long as new buds are 

 added to the coh)ny. 



Reinhard (9) studied the first budding in the embryo of Aleyondla 

 fungom and Cristatella mucedo. The cells formed by the segmontn- 

 tion of the ovum produce a true gastrula by invagination. Tlic 

 blastopore, however, soon closes. The gastrula is comparable in all 

 respects to the type of some other animals, and, therefore, he regards 

 the inner layer as the entoderm. In the development of tlie bud, the 

 entodermal cells seem to push into a certain thickened portion of the 

 ectoderm, and form a part at least of the wall of the alimentary canal. 



Salensky (11) also states that the outer layer of the zoœcium 

 (cystid) gives rise to the lophophore and to the internal cells of the 

 digestive tract, while the inner layer becomes the lining epithelium of 

 the new polypide. He believes that the entoderm of the alimentary 

 canal originates from the ectoderm of the zoœcium. 



Haddon (3) who studied the gemmation of some marine Polyzoa, 

 came to the conclusion, on theoretical grounds rather than from 

 actual observation, that the alimentary canal is derived from the 

 entodermic tissue of mother polypides. 



To the position and the order of budding, the previous 

 workers seem to have paid but little attention, except Bra^m who 

 dwells on the matter at some length. To this author we owe much 

 of the exact knowledge of the process of budding. As will directly 

 be seen, the process of budding takes place at certain definite poly- 

 zoöids and in a certain definite manner, thus determining the shape 



