n8 



HOLMES. 



[VOL. I. 



Ib* 



2c 



The next cleavage occurs in the macromere D, and results in 

 the formation of a yolk-laden cell, lying obliquely above the 

 larger stem cell in such a way as to indicate that the division 

 was laeotropic. This cell corresponds exactly as regards its 

 time and mode of origin with the primary mesoblast cell of 



other mollusks. The corre- 

 sponding division of the other 

 three macromeres to form the 

 remainder of the fourth quar- 

 tette does not occur until a 

 considerably later period. 

 These divisions do not give 

 rise to ectomeres, but to large 

 yolk-laden entomeres, the 

 cells of the fourth quartette 

 being somewhat larger, if 

 anything, than those at the 

 vegetal pole. 



About the time the pri- 

 mary mesoblast cell is given 

 off the four apical cells of 

 the first quartette divide in 

 a dexiotropic direction, the 

 outer products of the division 

 forming the basal cells of the 

 arms of the cross. Up to 

 this time the cleavages of 

 Serpulorbis agree, point for 

 point, with those of Crepi- 

 dula, Lymnaea, Limax, Pla- 

 norbis, and Physa, with the 

 exception that the divisions 

 in the latter two genera are 

 reversed. A comparison of 

 the forty-eight-cell stage, shown in Figs. 4 and 5, indicates 

 that the following divisions have taken place : The four 

 upper cells of the second quartette have divided in a laeotropic 

 direction, giving rise to the cells 2<7 1 - 1 , 2^ 1 - 1 , etc., which 



FIG. 4. Forty-eight-cell stage, seen from the ani- 

 mal pole. The outline of the cross is marked 

 with a heavier line. A dexiotropic twist is ap- 

 parent in the arms of the cross. The small 

 mesoblast cells are shown in dotted lines on the 

 posterior side of the egg. 



c 



M 



D 



M 



2-3 



Fir.. 5. Posterior side of the same egg, showing 

 the four derivatives of .)</, the upper pair budding 

 off the mesomeres, w 1 and in-, into the interior 

 of the egg. 



