1887] Metschnikoff on Germ-Layers. 429 
published, without referring to Balfour or myself, a repetition of 
the view that a close relationship existed between the ancestors 
of the Ccelenterates and Turbellaria (Accela), and that the endo- 
derm in the primitive Metazoa was in the shape ofa parenchyma. 
From this paper it may be inferred that the number of students, 
who feel themselves compelled to postulate a Phagocytella-like 
condition, is gradually increasing. The latest researches on the 
anatomy of the Accela (by.Kleinenberg, Pereyaslawzew, Yves, 
Delage, and myself) confirm the statement of Graff, that a me- 
soderm and nervous system are wanting in these Turbellaria. 
The fact also remains that the digestive organs of the Accela 
have preserved a very primitive condition, though it has recently 
been asserted that these worms possess an unmistakable digestive 
cavity. 
On the contrary, the true Aceela exhibit an endodermal plas- 
modium containing vacuoles of various sizes, which may imitate 
the appearance of a special digestive cavity. I was best able to 
make out these facts on a transparent pelagic form of great 
beauty, which I obtained at Messina (where it has been several 
times studied by Kleinenberg), and which fully convinced me of 
the truth of the statement just made. It appears from the em- 
bryological investigations of Miss Pereyaslawzew (35) and of 
Repiachoff (36), that in the Accela studied by them the seg- 
mentation is followed by a gastrula stage. The latter author 
concludes from this fact that the Accela are degenerated worms. 
But the formation of a gastrula is by no means to be uncon- 
ditionally regarded as a genealogically primitive process. In 
the development of the Medusz we saw that the gastrula (in one 
case as the archigastrula of Nausithoé and Pelagia, ip the other 
as the epibolic gastrula of Polyxenia leucostyla) might arise poly- 
phyletically from totally different methods of forming the endo- 
derm. It is quite possible, then, to regard the occurrence of a 
gastrula in the course of development as a secondarily acquired 
embryonic adaptation. Finally, I must remark that not until 
the work of Miss Pereyaslawzew and of Repiachoff has been 
published in full should an ultimate decision regarding the gas- 
trula of the Accela be expressed. 
Since my view supposes that gastrula forms may arise sinde- 
pendently in the course of embryonic development, on its adop- 
tion many difficulties encountered by the gastræa theory are 
