542 ALFEBD C. HADDON. 



derm, the inner epithelium as the inner germinal layer or 

 endoderm ; and if we further regard the tunica rauscularis as a 

 middle germinal layer or mesoderm, we may view the young 

 polypide-bud as composed of two concentric cellular layers, 

 the internal derived from the ectoderm, the external from the 

 endoderm of the cystid, while the mesoderm of the cystid takes 

 no part in the formation of the bud. . . Folds and secon- 

 dary introversions of this two-layered cellular sac give to the 

 young polypide its definite form. . . The inner epithelium 

 of the alimentary canal is derived from the ectoderm of the 

 cystid, while the outer is derived from the endoderm. The 

 two layers of the tentacular sheath have a precisely similar 

 derivation." A detailed account of the further development is 

 given, and on p. 501 we read, " as Nitsche suggests, we must 

 not in the present instance lose sight of the fact that the inner 

 layer of the bud, though arising from the ectoderm of the 

 cystid, has fundamentally different relations from those of an 

 ordinary ectoderm, for there proceeds from it, at the same time 

 with the nervous substance of the ganglion, the entire epithe- 

 lial lining of the intestinal tract." While the " endoderm " of 

 the cystid behaves in all respects like an ordinary Me soblas tic 

 tissue. What could have been the conditions which in 

 process of time have so upset the traditional functions of the 

 germinal layers ! 



Hatschek (8) describes, in Cristatella, the relations of the 

 polypides to the colony and the increase by gemmation. In 

 the lateral growing points he finds that there is a single- 

 layered sac, depending within the cavity of the stolon and 

 slung by a mesodermic layer. This sac constricts into two 

 unequal portions : the portion constricted off becoming the 

 inner epithelium of the alimentary tract of a new polypide, the 

 tentacular portions being supplied by an epiblastic involution 

 (Hatschek, fig. 3, p. 589). This process being repeated, 

 Hatschek says that Nitsche's figures {22 and 23) do not prove 

 his statement that in Alcyonella the inner layer of the poly- 

 pide sac is derived from the ectoderm of the cystid, and goes on 

 to say that Nitsche's figures will bear his (llat^chck's) inter- 



