ON Tili: i;a«tiiulatiün in i'ETuomyzox. li'> 



ij).), in wliicli the larger part of the archeiiteric root' is already 

 occupied hy the inicronieric layer. In tlu^ stage of an ovum 

 rei)resented in F'uj. 21, the anterior end of tlie arclienteroii 

 lias pushed its way across the segmentation cavity and has struck 

 against the ventral part of the peristomal mesoderm which is 

 brought forth from the ventral junction line destined to be 

 transformed into the ventral lip of the blastopore. 



The mitosis met with very frequently in the outer micromeric 

 layer {Figs. 19-21 and Figs. 27-30) has naturally no other 

 significance than an active growth of the layer, by which not 

 only the outer layer itself but also the archenteric roof is ex- 

 tended. 



At the stage shown in Fig. 21, however, a large part 

 of the macromeric hemisphere is still exposed. This part 

 invaginates, as before, by its own activity as shown by the 

 depression on the surface ; but in order to bring forth the 

 complete gastrula, this part is doubtless overgrown to a great 

 extent by the dorsal and lateral blastoporic lips now formed of 

 the micromeric ej^ithelium. The overgrowth is, however, certain- 

 ly brought about, not by a free growth of the lips' rim as it 

 were, but by the outer micromeric epithelium itself which is shift- 

 ing backwards uninterruptedly from earlier stages and is extended 

 by the active superficial growth mentioned above. This layer of 

 course migrates inwards about the blastoporic lips, but, at the 

 same time, it narrows the blastoporic aperture, causing the lips 

 to grow on in such a manner that the blastopore is ultimately 

 converted into the definitive circular form (Fig. 22). 



From the above observations, we see that the micromeric 

 epithelium is brought into the archenteric roof by its infolding 

 and that it overgrows a large part of the macromeric hemisphere 



