134 CONK LIN. [Vol. XIII. 



provisionally as 2b^% and which lies just beyond the median 

 cells of the first row, Figs. 56, 69, 70. I have not been able 

 to determine whether any part of the second row arises by sub- 

 division of the cells of the first ; if not, this row may include 

 a few cells of the third quartette (3a'-'-' and 3b'-'-S Fig. 56) 

 at the points opposite the anterior turrets. 



These are the only velar cells whose origin I have been able 

 to determine with any degree of probability ; even in the case 

 of these I recognize that there is an element of uncertainty, 

 since the lineage was not followed cell by cell to a later 

 stage than Figs. 47 and 48. However, I hold it highly proba- 

 ble that my identification of the velar cells in Figs. 50 and 56 

 is correct, and the identification in the later stages. Figs. 65 

 et seg., is only a little less probable. 



Several irregular rows of cells intervene between the first 

 row of velar cells and the mouth, and in the latest stages 

 figured several rows are seen running posterior to the mouth. 

 All of these cell rows can be traced outward to the sides of 

 the embryo, and all of them are ultimately ciliated and form 

 part of the velum. Most of these cell rows could not have 

 come from the first and second velar rows, and they must there- 

 fore have been derived from cells lying still farther away from 

 the apical pole. 



Thus the preoral velum is composed of a few cells of the 

 first quartette, many of the second, and possibly a few of the 

 third. It consists of many rows of cells, more or less regular 

 in arrangement, extending from the first velar row in front to 

 the edge of the mouth behind. Figs. 79, 81, Si2. Of course 

 the postoral velum must be composed of still more remote cells 

 of the second and perhaps even of the third quartettes. 



From the time of their appearance, Fig. 65, the first and 

 second velar rows are slightly curved forward on the ventral 

 mid line. In later stages. Figs. ^6 et seg., when, with the 

 development of the foot and shell gland, the mouth is moved 

 forward from the middle of the ventral face, this middle por- 

 tion of the velum is carried still farther forward and at the 

 same time the lateral portions of these velar rows are elevated 

 above the general level and finally drawn out into a pair of 



