J 



86 HOLMES. [Vol. XVI. 



From the Twenty-fotcr to the Forty-nine Cell Stage. 



The Division of the Second Quartette and the Formation of 

 the Cross. — The transition from the twenty-four to the forty- 

 nine cell stage occurs very quickly. The upper tier of the sec- 

 ond quartette divides in a dexiotropic direction. The upper 

 cell, as in Crepidula, Umbrella, and Unio, is the smaller, and 

 Meisenheimer's Fig. 31 shows the same is true also in Limax. 

 These upper cells, as in the above forms, form the tips of the 

 arms of the cross presently to be described. Blochmann was 

 doubtless wrong in his derivation of these tip cells in Neritina, 

 and I think Conklin's correction of this mistake is to be fol- 

 lowed, rather than that given by Kofoid, as it brings the cleav- 

 age of Neritina into complete harmony with that of other 

 moUusks (see Conklin ('97), p. 64). The tip cells of the lateral 

 arms of the cross in Neritina were found by Blochmann to have 

 a peculiar granular appearance, and were held by him to give 

 rise to the velum. It is probable, however, that only a portion 

 of the velum is formed from these cells, as in both Crepidula 

 and Planorbis, and also in Ischnochiton, certain cells of the first 

 quartette, and other cells from the second also, go into the 

 formation of this organ. The granular character of these tip 

 cells in Neritina has not, I believe, been seen in any other 

 form. 



The division of the upper tier of the second quartette is soon 

 followed by that of the lower. This cleavage is also dexio- 

 tropic, but the smaller cell is now the lower one. The upper 

 cell, resulting from this division, lies at the side of the lower 

 cell, arising from the previous division. There are now four 

 groups of four cells each, or sixteen cells, in the second quar- 

 tette. In each group there is a pair of large cells situated side 

 by side, and a smaller cell above and one below. The lower 

 cells lie opposite the entomeres. 



The apical cells of the first quartette now divide in a laeo- 

 tropic direction. When this division is completed, the arrange- 

 ment of certain cells of the upper pole becomes such as to give 

 the appearance of a cross. The outer cells of the first quar- 

 tette, Ia'■^ i<^'^ etc., form the bases of the arms of the cross, 



