28 ART. n. — s. H ATTA. 



the infolding micromeric epithelium {e. mic. ep.). In the 

 third figure {Fig. 29), the already infolded epithelium forms 

 the median part of the archenteric roof ii. mic. ep.) which is 

 obliquely cut through. It is therefore obvious that so long as 

 the macromeric field is exposed on the external surface, the 

 blastoj)oric lips on the lateral sides, i. e., on the limbs of the arc, 

 stand apart from each other ; when the macromeric half is 

 entirely withdrawn inside and the infolding of the micromeric 

 epithelium commences its work, the lips are brought nearer : 

 hence resuhs the change of shape in the blastoporic arc. I 

 should here add that neither the micromeric nor the macromeric 

 part is passive in the process, but both are in active state. The 

 infolding of the former is, of course, carried on by its own 

 activity, l. e., by extension of the layer in general, which is 

 brought about by both the cell-multiplication of the epithelial 

 components and the dovetail-insertion of the micromeres forming 

 the cupola of the segmentation cavity. The invagination of the 

 macromeres is likewise actively taking place ; this is in fact 

 the same active process as that which more early caused the 

 blastoporic depression. 



At the stage of Fi(/. VJ there is already comparatively little 

 cell disturbance in front of the archenteron {Fiff. 19, ■::••::• ). This 

 shows that the delamination of the outermost cell-layer of the 

 macromeric mass to form the archenteric roof is of a short dura- 

 tion and of a limited extent. On the other hand, there is a 

 strong reason, as briefly stated on p. 13, for believing that the 

 backward shifting of the outer micromeric layer {Fl(/. 1*.>, e. mic. 

 c)).)y at the cost of which the formation of the archenteric roof 

 is accomplished, is of a greater importançLe. This shifting seems 

 to be more active in later stages {Figs. liO, 21, and 24, i. mic. 



