2 54 



I. TKEDA : 



the divisions of 2a.,, etc. into 2a n and 2a ±1 , etc., which lead the egg 

 into the 52-cell stage. As this cleavage goes on quite similarly, 

 — e. i. laeotropically — in the four quadrants, there appears no trace 

 of asymmetry to determine the subsequent bilateral symmetry. The 

 stage consisting of 52 cells is rather transient, since the cells 2a x , 

 etc. very soon follow; the above cleavage to divide slightly unequally 

 into 2a n and 2a vl , etc. 



The formation of the fourth quartet (4«, etc.) takes place at the 

 56-cell stage, while the first division in the third quartet (3«, etc. into 

 3^ and $a, , etc.) occurs at the 60-cell stage, thus giving rise to the 

 64-cell stage. These cleavages are likewise effected by equal and 

 laeotropic division, and accordingly do not give any determination 

 to the future bilateral symmetry. I have observed some eggs consist- 

 ing of 72 cells. They have probably arisen from the 64-cell stage 

 by dexiotropic division of such cells in the first quartet as ia m , 

 etc. (the rosette) and ia m etc. — the latter corresponding to the stem- 

 cells of the " cross " in polychaete eggs — into ia im and ia im etc, 

 ia nn and ia ni < etc. But I cannot be certain on this point, since I have 

 not actually detected any division-figures causing the supposed cleavages. 



To recapitulate, the segmentation-process belongs, as in the case 

 of Cerebratulus lacteus observed by Wilson, to a very regular spiral 

 type and shows in its earlier phases no directive cleavage determin- 

 ing the axes of embryo and the origin of mesoderm. As was 

 pointed out by Wilson in his case, the cross formation takes place in the 

 present case also at least not earlier than in the 72-cell stage. Though 

 I cannot make any definite statement about the origin of the mesoderm, 

 yet it seems almost certain to me that Lebedinsky's view of the origin 

 of mesodermal bands from four mother-cells does not hold good in the 

 present case. Here, the mesodermal bands are present in a pair as 

 was observed by Wilson also, and are produced by two large cells 

 which lie symmetrically one on each side of, and near the postero- 

 lateral lip of, the gastral invagination. 



