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• ALFRED GIARD. 
In the Ophiurids with condensed emhryogeny, the egg 
presents an enormous nutritive yelk, and the mesoderm is 
formed by abbreviation at the same time as the endoderm 
by a general delamination of the ectoderm, which leads us 
to Metschnikoff’s form dubbed Parenchymula, This I have 
established most clearly in cases of Ophiocoma neglecta and 
Ophiothrix fragilis. 
It is clearly impossible for me to discuss here the im- 
portant questions of general embryogeny which are raised by 
the study of the Orthonectida. I will merely say that, 
according to my observations, a large number of calcareous 
and siliceous sponges present what one may well call a 
biconvex archigastrula. Quite recently Keller has shown 
that Chalinula possesses an amphigastrula. Kowalevsky 
has described, in the most exact manner, the existence of an 
archigastrula in a variety of Actinia mesemhryanihemum, 
in Cereanthus, and in various Medusae. I am able to confirm 
his statement in reference to Actinia equina. Finally, Ed. 
van Beneden has described and figured an amphigastrula 
in the Dicyemida, so closely related in many respects to the 
Orthonectida. 
These examples will suffice, I think, to justify my opinion 
that the gastrula by invagination is the primitive mode ; 
the gastrula by delamination [Planuba or Parenchymula) a 
secondary mode of embryonic development. 
I wmuld also direct attention to the metamerisation which 
is so remarkable in the Orthonectida, 
We have seen that this metamerisation only affects the 
ectoderm, and I believe that this was primitively the case 
also in the Annelids. What proves this to be the case is 
the highly differentiated form of the digestive tube of the 
Chsetopods in which metamerisation is only well marked in 
those organs derived from the ectoderm, such as the bristles, 
the parapodia, and the segmental organs (nephridia). 
It is obvious enough that I am not alluding to that kind 
of metamerisation which is observed in Salmacinay Syllis, 
&c., which is merely the result of gemmiparous reproduc- 
tion. This last kind of metamerisation is only comparable 
to what one finds in the Cestoidea and the Rhabdocoela. 
