MUSEUM OF COMPAKATIVE ZOOLOGY. 95 



to Braem's ('90, Taf. VI L Fig. 89 mh.) observations, which I can abun- 

 dantly confirm, — and to the coelomic epithelium of the adult stock. In 

 the few series of sections of the proper stage which I possess, I have not 

 found with certainty the degenerating cells of which Korotneff speaks ; 

 but even if they regularly occur, I should be inclined to regard them as 

 the degenerated entoderm, the mesoderm persisting to give rise to the 

 muscular tissue and the coelomic epithelium. 



From a consideration of these facts, — that the larvse are homol- 

 ogous and the process of gastrulation is comparable throughout the 

 Ectoprocta, that in the least modified larvse both functional entoderm 

 and mesoderm are produced by that gastrulation, that one of these 

 two germ layers has become rudimentary in Phylactolcfimata, that 

 it is highly probable that the entoderm has disappeared from loss of 

 function, and that the layer which persists gives rise to the muscula- 

 ture, sexual cells, and coelomic epithelium, — I conclude that the inner 

 layer of the Phylactolcematoiis larva, and therefore the outer layer of the 

 hud, is mesoderm. 



If we accept the point of view of Kleinenberg ('86, pp. 1-19) and ad- 

 mit the existence in general of only two layers, ectoderm and entoderm, 

 a clearer conception of the modification undergone by the Phylactolsema- 

 tous larva may he gained. We may divide the entoderm arising in 

 Bryozoa into two parts ; viz. (1) that which gives rise to the lining 

 of the midgut, as in Cyphonautes, and (2) all the rest of the inner 

 layer. Now, since no midgut is formed in the Phylactolaematous larva, 

 part (1) of the entoderm has ceased to be differentiated; all which 

 remains, then, is part (2) ; but this is equivalent to " mesoderm " in 

 the sense in which I have employed it, and therefore I am justified 

 in saying that "mesoderm" only is produced. 



The question has now to be answered, What is the significance of the 

 inner layer of the hud ? Two different answers have been given to this ques- 

 tion. It has been maintained, on the one hand, that it is to be regarded' 

 as ectoderm ; on the other, as entoderm. There are serious difficulties 

 in the way of accepting the first view, — so serious, in fact, that few 

 authors have maintained it, although at first glance it seems to be re- 

 quired by the facts. Although we have not yet sufficient grounds for 

 declaring that organs formed by budding must be built up from the 

 same germ layers as corresponding larval ones, — although we may ad- 

 mit that gemmigenesis recapitulates phylogeny and coriesponds with 

 ontogeny only in an imperfect and conf\ised way, — still, from the expe- 

 rience gained by tracing the development of hundreds of animals from 



