158 THF ,. ?LASM 



it unlikely that any oversight as regards the migration of cells 

 can have occurred. 



B. — Polysoa 



The^small stocks or colonies formed i n the P ol yzoa a rise by 

 ajjrocess of gemmation ; and even the small number of species 

 which do not form stocks multiply vigorously by budding, but 

 in these cases the buds become detached from the parent sooner 

 or later. 



The process of gemmation seems to be essentially similar in 

 all Polyzoa. A proliferation, which primarily originates in o)ie 

 cell, takes place in a certain region of the epidermis ; the masses 

 of cells which are thus produced form a hollow invagination, 

 which extends into the body-cavity of the animal and gives rise 

 to the entire alimentary canal — including the fore-, mid-, and 

 hind-guts, as well as to the preoral 'atrium' with the tentacles 

 ('lophophore'). Certain 'free mesoderm cells' are then said 

 to migrate from the body-cavity of the parent into the bud, in 

 which they give rise to the muscles and sexual organs, and also 

 in certain groups of Polyzoa to the outer (serous) layer of the 

 intestine ; while in others again they form a subcutaneous layer 

 of cells. This is at any rate the case according to the recent 

 observations made by Seeliger,* which are undoubtedly very 

 accurate and trustworthy. But one point, however, still seems to 

 be doubtful, viz., whether the sexual organs may not perhaps, after 

 all, arise from the primary proliferation of the epidermic cells. 



These processes of gemmation interfere very considerably 

 with the ordinarily accepted and extremely conventional ideas 

 of the germinal membranes ; for the epithelium of the alimentary 

 canal, which characteristically belongs to the inner germinal 

 layer, here arises from the ectoderm. This, however, causes no 

 difficultv from the point of view of the theory of the germ-plasm : 

 we need only assume that the group of determinants for the 

 endoderm is passed on to certain cells of the epidermis as acces- 

 sory idioplasm. This transference must take place at an early 

 stage in embryogeny. before the separation of the primary endo- 

 derm and ectoderm occurs. 



Nitsche, whose means of observation were comparatively im- 



* O. Seeliger, ' Die ungeschlechtliche Vermehrung der endoprokten 

 Bryozoen," and ' Bemerkungen zur Knospenentwicklung der Bryozoen," 

 Zeitschr. f. wiss. Zool., Bd. 49 and 50, 1889 and 1890. 



