TROCHOPHORE OF HYDKOIDES UXCIXATTS (k L'POMATU.s). 549 



tlioug-lit, direct from the ventral side of the ectoderm. This 

 was followed by the work of AVhitmau (45) on Clepsine, 

 Bergh (3) ou Lumbricus, Yejdovsky (44) ou Oligochsets. 

 Schimkewitsch (32) in Diuopliilus described a separate 

 origin of the mesenchyme in the anterior end of the larva 

 from the definite mesoderm of the posterior region. Finally 

 the separate nature of mesenchyme and coelomesoblast has 

 been most ably championed in the very extensive researches 

 of Meyer (27) on the mesoderm of Annelids. 



In the work of the cell-lineiige investigators, however, the 

 distinction between larval and coelomesoblast has been most 

 definitely brought to light. In all x\nnelids, Lamellibranchs, 

 and Gasteropods studied by them, with one exception, the 

 coelomesoblast iuvariably arises from a large cell in the 

 ventral side of the blastophore {4d). The one exception is 

 the Annelid Capitella, where, according to Eisig (11), it 

 arises fi-om the third and fourth quadrants of the third 

 ([uartette. Here the cell 4d contains a little larval mesoblast, 

 but the main portion contains ectoderm. In Molluscs, accord- 

 ing to Conklin (7), 4d, while containing the coelomesoblast, is 

 more than half eudoderm. In the Annelid Podarke, accoi-d- 

 ing to Treadwell (42), 4d divides and then sinks in, and 

 takes up its position in the endoderm of the archenteron 

 (Text-figs. 4, 5, 6). Here at a later stage it gives rise to the 

 coelomesoblast. 



At the time 4'1 is being invaginated, or even before, 

 irregular ectoderm cells are given off into the interior of the 

 blastococl ; these are the larval mesoblast cells. They 

 migrate inwards and scatter throughout the cavity. Their 

 origin has been determined in a large number of forms, 

 first by Lillie (25) in Unio, and then by Conklin (7) in 

 Crepiduia, Treadwell (42) in Podarke (Text-fig. 5, I.m.r., 

 l.m.L, l.m.rii.), Wierzejski (46) in Physa (Text-fig. 11, I.m.r., 

 l.iii.l.),Torrey (41) in Thalassema (Text-figs. 8 and 9, I.m.r., 

 l.m.L, l.m.m.). The mode of origin of the ectomesoblast, there- 

 fore, is distinctly in opposition to that of the ccelomesoblast. 

 In Unio it arises asymmetrically, and only afterwards takes 



