392 WILSON. [Vol. VI. 



6. TJiird Spiral Division of the Pj'imary Micromeres ; Formation 

 of the Apical Rosette ; Of'igin of the Second Soniatoblast ; 

 Second Division of the First Soniatoblast. 



The strictly spiral period of the cleavage is brought to a close 

 by three events. 



(i) The four primary micromeres («^ b^, c^, d^) bud forth four 

 small cells at their inner angles (at the upper pole), which 

 arrange themselves in a very regular apical rosette, the cells of 

 which alternate with the central micromeres (Figs. 27, 28, etc.). 

 The position of the spindles is the same as in the first division 

 of a^, b^, c^, d^ ; i.e. the division follows a right-handed spiral, 

 but the character of the division is very different, since the 

 smaller cells are formed at the central instead of the peripheral 

 angles of the cells (i.e. towards instead of away from the verti- 

 cal axis of the embryo). In this case the spiral character is 

 accentuated by the fact that the cells divide in the following 

 regular order : c^, d^, a^, b^. I have observed this several times 

 in the living embryo, and I have also obtained several prepara- 

 tions like Fig. 27, in which the order of division is completely 

 shown by the nuclear figures. {Cf also Figs. 25, 26.) At a 

 later period the apical tuft of cilia appears exactly at the point 

 where the rosette is formed, and there can be no doubt that it 

 arises, in part at least, from the rosette-cells. I have not been 

 able, however, to trace the connection in detail. An almost 

 exactly similar rosette is formed in the polyclade Eiuylepta 

 (Selenka, No. 22), but its fate is doubtful. 



(2) During the formation of the rosette (or sometimes some- 

 what later) the somatoblast buds forth at its left apex a small 

 cell {x^) which lies symmetrically with x^ (Figs. 25, 29, 31). 



(3) After a short pause, a large granular cell {M, Figs. 29, 31, 

 33, 34), somewhat smaller than X, is budded off obliquely from 

 the left posterior macromere {D). This cell is the second soma- 

 toblast or primary mesoblast, from which the mesoblast-bands are 



formed. It lies below X (i.e. anterior to it), and considerably 

 to the left of the median line {i.e. the cleavage-line between C 

 and D). Its exact position and mode of formation may be seen 

 by a comparisot| of Figs. 29, 31, 33, 47, and 48. 



The formation of the second somatoblast ends the spiral 

 period of development, and it is a very significant fact that the 



