30 BIOLOGICAL LECTURES. 



this evidence the two principal difficulties should be clearly 

 borne in mind. The first lies in the fact that the mesoblast- 

 bands of the annelids and mollusks arise from one cell of the 

 fourth quartet, while in the polyclade the mesoblast was stated 

 to arise from all of the eight cells of the second and third quar- 

 tets. The second difficulty relates to the ectoblast, which in 

 the annelid and mollusk arises from the twelve cells of the 

 first, second, and third quartets ; while in the polyclade it was 

 believed to arise solely from the first quartet (Fig. 2). We 

 may consider these two difficulties in order. 



As regards the first point, a series of researches during the 

 past three years have shown that in some of the mollusks and 

 annelids the mesoblast has a double origin, a part and usually 

 the major part arising from the posterior cell of the fourth 

 quartet, as stated above, while a part arises from cells of the 

 second or third quartet, as in the polyclade (Fig. 3). The major 

 part which, for reasons that will appear beyond, I propose 

 to call the entomesoblast gives rise to the so-called mesoblast- 

 bands. The minor part, or ectomesoblast (" secondary meso- 

 blast," "larval mesoblast," of various authors), apparently does 

 not contribute to the formation of the mesoblast-bands, and 

 in at least one case namely, that of Unio, as described by 

 Lillie it gives rise to cells of a purely larval character and 

 designated as " larval mesenchyme." The first step in this 

 direction was that of Lillie, just referred to, who in 1895 an- 

 nounced the discovery that in a lamellibranch, Unio, one ceil 

 of the second quartet (a 2 on the left side) gives rise not only to 

 ectoblast, but also to a single mesoblast-cell which passes into 

 the interior, divides, and gives rise to some of the larval mus- 

 cles ("larval mesenchyme," Fig. 3, C). Lillie's discovery was 

 quickly followed by the no less interesting one of Conklin that 

 in another mollusk, the gasteropod Cre'pidula, three cells of the 

 second quartet, median anterior, right, and left (b 2 , c 2 , d 2 ), like- 

 wise give rise to mesoblastic as well as to ectoblastic elements 

 (Fig. 3, B), a process still more forcibly recalling the origin 

 of the mesoblast in the polyclade. 



Two years later mesoblastic cells were found, both in the 

 mollusks and in the annelids, to arise from members of the 



