42 M. H. de Lacaze-Duthiers on the Develojjrnent 



would be useful at starting to give a precise histological defini- 

 tion of the dermis, which has never been done. Now-a-days 

 two layers are distinguished in the body-walls of polypes, an 

 inner and an outer one, called the ectoderm or ectothelium and 

 the endoderm or endothelium. French authors have spoken of 

 these two layers ; but they have subdivided them into numerous 

 secondary layers separated by a plane of muscular fibres. It 

 is therefore outside this muscular layer that the primitive de- 

 position of the part which in their eyes is the most important 

 takes place, namely that which forms the walls of the calice 

 [iheca). 



Now the embryogeny and histology of the embryo, studied 

 in young living Astroides of all ages, and not in polyparies at 

 some given moment of their existence deprived of their soft 

 parts, show Avithout any possibility of doubt that the first cal- 

 careous nodules occur and appear in the inner layer or endo- 

 thelium^ the histological characters of which are absolutely dif- 

 ferent from those of the outer layer, which does not allow us 

 to confound them. 



Thus, as regards the histological origin of the polypary, it 

 is impossible to continue to accept the old opinion, and con- 

 sequently also the denomination Sclerodermi. 



With regard to the law governing the mode of appearance 

 of the septa of the polypary, the following appears to us to be 

 no less certain. The primitive calcareous nodules first de- 

 posited, making their appearance in the thickness of the inner 

 layer, clothe the bottom of the cavity of each chamber of the 

 embryo while it is still without tentacles, and unite together 

 to form usually a central band at the bottom of the chamber, 

 this band being simple towards the middle of the body and 

 bifurcate towafds the circumference ; so that at one moment 

 we find at the bottom of each chamber a sort of calcareous Y, 

 the branches of which turned outwards may be either very 

 short or very long. 



It is to be remarked that at this period there is no trace of 

 any circumvallation or wall (theca), or exterior boundary of 

 the calice. 



By following these first deposits we find that they rise more 

 and more beneath the inner layer, and that filling up the fork 

 of the Y they produce projecting simple laminae, one in each 

 chamber of the embryo. These lamina (the origins of the 

 septa) become soldered to foreign bodies underlying the em- 

 bryos, and constitute the first rudiments of the polypary. 



Now, there are twelve chambers ; there are consequently 

 twelve primitive septa, and, I repeat, no wall. Nevertheless, 

 by the examination only of the polyparies in collections. 



