EMBRYONIC FISSION IN CYOLOSTOMATOUS POLYZOA. 225 



Ostroumoff^s pi. vi, fig. 2, which, by the way, are unlike any 

 cells which I have ever seen in a Cyclostome larva. 



Whatever be the origin of the outer layer of the bud which 

 which forms the primary polypide^ it is quite clear that that 

 polypide is formed in fundamentally the same way as any other 

 polypide in the future colony. There can be no question of 

 the '^alimentary canal" of the embryo passing over directly 

 into that of the primary zocecium. 



In default of sufficient evidence on this point I am inclined 

 to regard the inner layer of the Cyclostome embryo as meso- 

 dermic rather than endodermic, and this principally on the 

 following grounds : 



1. The alimentary canal is an excessively rudimentary struc- 

 ture in the great majority of known Ectoproct larvse. 



2. The peculiar character of the early development of 

 Crisia suggests that a representative of this rudimentary 

 structure is likely to be found in the primary embryo only, 

 and that the secondary embryos, formed by budding from the 

 primary one, are no more likely to possess an alimentary canal 

 than is a young zooicium formed at the growing-point of an 

 old colony. 



3. The analogy of other Ectoprocta is in favour of this hypo- 

 thesis.^ 



Prouho (26), for instance, has given an account (which I 

 can confirm in the main from my own observations) of the 

 metamorphosis of Flustrella. Even before the end of larval 

 life, a distinct aboral mesodermic layer is present, from which 

 the outer layer of the bud is directly developed. 



In the course of the budding of an ordinary Ectoproct 

 colony the polypide buds are formed from two distinct layers. 

 The inner layer of the bud is developed at the expense of the 

 ectoderm; the outer layer, either from an already definite 

 layer of mesoderm (Phylactolsemata), or from mesoderm-cells 

 of the funicular tissue which arrange themselves as an epithe- 

 lium round the outside of the ectodermic portion of the bud 



1 Cf. particularly the larva of the Phylactolseniata, as described by Braem 

 (5) and by Davenport (8a). 



