1884.] 127 [Hyatt. 



the epiderm consists of several layers. A corresponding trans- 

 ference of the planes of fission from approximately vertical diag- 

 onals to approximately horizontal diagonals is also figured in the 

 same treatise. It is evident that the planes of fission of the 

 amphimorula must have been radial or perpendicular to the sur- 

 face, and perpendicular and parallel to the long axis of the body > 

 thus being in accord with Kollmann's law in so far as the result* is 

 an increase of the body in length and breadth. On the other 

 hand delamination is an increase of the ectoblast in thickness and 

 here again we find the planes of fission parallel both to the sur- 

 face and to the axis of the body also, agreeing with Kollman's 

 law. 



Kollmann's and Gardiner's results are pertinent and can also 

 be applied to explain the transformations of the epithelium in 

 sponges. The cells of the ectoderm in Porifera pass through the 

 amoeboidal, wedge-shaped, columnar, and cylindrical forms. 

 During the last two stages they acquire llagella and collars in 

 most forms, but finally terminate in an epithelium of flattened 

 cells. The major axes of the cells are shifted from a radial in the 

 segmental wedges to a tangential direction in the flat epithelium, 

 and in each case they are at right angles to the direction of the 

 pressure due to growth of the neighboring cells. The excess of 

 growth in the embryo is invariably in the ectoblastic layer, and 

 this occasions the flattening and radial elongation of the cells in 

 this membrane until the completion of the cinctoplanula stage ; 

 then the excess of growth is transferred to the mesenchyme and 

 this increasing in thickness causes the flattening of the cells on 

 the exterior in the older stages. The Ascones have an exceedingly 

 thin mesoderm, but nevertheless an ectoderm is present of exces- 

 sively flattened cells. No one, however, can study this in living 

 specimens without seeing that considerable pressure is exercised 

 by the stiff spicules which support the body, and that these parts 

 must be taken into the account of the causes which may occasion 

 the 'flattening of the outer layer. The functions of assimilation 

 being maintained by the endoderm throughout life in such forms 

 as Ascones, the cylindrical cells are retained also, together with 

 their hereditary protozoonal collars and flagella, even in adults. 

 When, however, the function and pressure are transferred to 

 the ampullae, as in Sycones, we find that flattened cells appear in 



