56 PROFESSOR VAN BENEDEN. 



one is only the intermediate layer, in the deepest part of which 

 the cells are disposed as a simple pavement epithelium. Between 

 the two exists an incomplete layer of cells which are very dif- 

 ferent. This layer is absent from the vault of the roof of the 

 segmentation cavity, and its thickness diminishes progressively 

 from without inwards. It is evidently this layer which Haeckel 

 considered to be formed by the invagination of the margin of the 

 blastodisc and as representing the endoderm. \Yhat is the 

 origin of this layer? Is it derived from the blastodisc, or, on 

 the other hand, is it composed of cells derived from the interme- 

 diate layer ? I believe that it is composed of cells which origi- 

 nate in the peripheral welt of the intermediate layer. I base this 

 opinion on the following considerations : 



1st. The cells of this layer have the same dimensions, the same 

 form, the granular texture, the same oval nuclei, sometimes 

 irregular and always nucleolated, which we observe in the cells 

 found in the substance of the peripheral welt, and which are, 

 ■without doubt, of endodermic origin. They are, on the other 

 hand, very different from the cells of the blastodisc. 



2ndly. The cells which rest on the floor of the segmentation 

 cavity, and which are formed at the expense of the "median 

 lens " of the intermediate layer, possess the same characters as 

 those which form the middle layer. 



3rdly. The blastodisc remains all this time very sharply de- 

 limited infcriorly, and in no part is there a passage from one to 

 the other. In no part have I found the slightest indication in 

 favour of invagination. 



According to my view, the intermediate layer not only fur- 

 nishes the epithelium of the digestive canal, but it intervenes 

 largely in the formation of the middle layer, to which it probably 

 furnishes the connective and vascular elements. In the phase 

 represented in fig. 6 we must then distinguish — 



1st. The enveloping lamella. 



2nd. An ectodermic layer derived from the blastodisc, and 

 destined to subdivide subsequently into sensorial lamella and 

 external mid layer (first and second secondary blastodermic layers 

 of Haeckel) . 



3rd. Of an internal mid layer, of endodermic origin, destined 

 to furnish the vessels and the connective tissues (third secondary 

 blastodermic layer of Haeckel). 



4th. (3f an internal layer, destined to furnish new cells to the 

 internal mid layer and to give rise to the epithelium of the diges- 

 tive tube (fourth secondary layer of Haeckel). 



