78 Jenkinson, Germinal Layers of Vertebrates. 



atrium of the parent (Botryllidae), in which case it is 

 ectodermal, or may arise from the dissepiment of the 

 stolon {Clavellina, Didemnum), in which case it is a 

 derivative of the epicardium, that is to say of an organ of 

 endodermal origin. Now, in spite of this diversity, the 

 behaviour of the inner layer is by the concurrent testimony 

 of Seeliger, Oka, Pizon, and Hjort^^ essentially the same 

 in all cases ; it gives rise, in fact, to the pharynx and 

 alimentary canal, to the atrial cavity, and, according to 

 Hjort, also to the hypophysis and nervous ganglion of the 

 bud,though a different mode of origin of the last-mentioned 

 is described by the other observers. While therefore in 

 the first case an organ, the alimentary tract, which in the 

 embryo is derived from the endoderm, is in the bud of 

 ectodermal origin ; in the latter organs, the atrium and 

 nervous system, which are ectodermal in the larva are 

 developed in the bud from the other layer. 



The relation of gemmation to the germ-layers in the 

 Ectoproctous Polyzoa is just as great a source of per- 

 plexity. After segmentation, a two-layered stage is 

 reached, either by unipolar immigration or a process 

 wliich is reducible to this. The inner layer so formed — 

 comparable as far as its origin goes with the primary 

 endoderm of other forms — never takes any part in the 

 development of the adult alimentary canal ; although in 

 those few cases amongst the Gymnolaemata in which 

 the larva possesses a gut, this organ apparently arises 

 from these cells, the remainder becoming mesoderm. 

 Where the larva has no gut most of this inner mass 

 disintegrates and is ultimately found, together with the 

 degenerating larval organs, in the ' brown' body ; ' a small 



** Seeliger, O., Sitzungsb. Akad. Wiss. Wien, voj. 85, pt. i, p. 361 — 413, 

 1882. 



Oka, A., Zeitschi: wiss. ZooL, vol. 54, p. 521 — 547, 1892. 



Pizon, M. A., Ann. Sci.Nat. (Zool.), {•/), vol. 14, p, i — 386, 1893.. 



Hjorc, J., AnaL Anz., vol. 10, p. 215 — 229, 1895. 



