No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 411 



General Considerations on the Secondary Mesoblast, 



The origin of the secondary mesoblast in the Mollusca is a 

 subject to which, for many reasons, considerable interest is 

 attached. We possess, however, at present very few accounts 

 of the process by which the secondary mesoblast arises. The 

 first case in which a double origin of the mesoderm has been 

 carefully and accurately traced is that of Unio, studied by Dr. 

 Lillie. In this form both the primary and secondary mesoblast 

 are segregated at an early period. The primary mesoblast arises 

 from 4^, as in other mollusks ; the secondary or " larval meso- 

 blast," as it is called by Lillie, arises asymmetrically from a 

 cell of the second quartette, 2a^, on the left side of the Q.gg. 

 This cell is gradually overgrown by the surrounding cells, and, 

 after budding off two or three small cells to the surface, comes 

 to lie entirely in the blastocoel. Although the secondary meso- 

 blast arises asymmetrically, it afterwards becomes disposed in 

 a symmetrical manner by the migration, apparently, of some of 

 the cells to the opposite side of the Q.^g. The larval mesoblast 

 forms a kind of mesenchyme, which gives rise to certain larval 

 structures, which disappear in later development, and its early 

 segregation appears to be correlated with the early development 

 and importance of these organs in larval life. It is a significant 

 fact that the cell from which the larval mesoblast arises is larger 

 than the corresponding cell on the other side of the &^%. 



Secondary mesoblast was discovered later in Crepidula by 

 Conklin. It was found to arise near the edge of the blastopore 

 in the three quadrants, a, by and c, in which no other mesoblast 

 was produced. The exact cell origin of this mesoblast Conklin 

 was unable to trace, owing to the large number of cells in the 

 ^gg at that stage, but, from the position of the mesoblast cells, 

 it was shown to have arisen from the cells of the second quar- 

 tette. There is an anterior mesoblast cell in the b quadrant, 

 and a right and left cell bilaterally placed in the a and c quad- 

 rants. The d quadrant produces no secondary mesoblast unless 

 at a'very much later period. The mesoblast in Crepidula arises, 

 therefore, in each of the four quadrants. 



The secondary mesoblast in Physa, according to Wierzejski, 



