252 



oil analogy with other aiinehds, for Conn^) has found that in the 

 post-trochophore development of Thalassema they increase rapidly in 

 size and become segmented, just has been described by Hatschek for 

 Echiurus and by other investigators for various other annelids. 



Let us now turn our attention to the origin of the ectomesoblast 

 with which the trochophore is very abundantly supplied. In other 

 forms, thus far investigated, the so-called "larval mesenchyme" has 

 been described as arising from only one quartet (either the second or 

 third) whereas in Thalassema, as has already been indicated, all of the 

 first three quartets of ectomeres participate in its production. In all 

 there are at least 20 primary cells of this character, but of them only 

 10, arising from the first and third quartets, develope into functional 

 mesenchyme, while at least ten degenerate and are finally 

 absorbed by the entoblast cells. 



The main source of the functional ectomesoblast is in three cells 

 derived from the third quartet, which are almost identical in cell lineage 

 with those that produce the ectomesoblast in Podarke, according to 

 Treadwell. a short time before gastrulation a large cell from each 

 of the anterior, right, and posterior quadrants (a, c and d) sink into 

 the segmentation cavity (Fig. 2). The cells of the c and d quadi-ants 

 lie on each side of the Jf, M cells and sink in somewhat earlier than 

 the latter (Fig. 2, A). At the time of gastrulation they begin to bud 

 off small cells toward the M cells, dividing exactly simultaneously and 

 after the fashion of teloblasts, but budding backwards instead of for- 

 wards (Figs. 2, -B; 3, J., jB, C). The small cells they produce could 

 easily be mistaken for a part of the mesoblast-bands unless their cell 

 lineage had been carefully studied. As they bud they migrate to a 

 position on each side of the stomodseum and there elongate, forming 

 in all probability a part of the large ventral oesophageal muscles. The 

 small cells they have produced give rise to all the mesenchyme of the 

 subumbrella. 



The third cell, namely, that derived from the a quadrant of the 

 third quartet, behaves almost exactly like the corresponding cell in 

 Podarke. It divides equally as it sinks in, and the product of this 

 division nearest the ventral side migrates over toward the h quadrant 

 and produces the larval mesoblast in that region (Figs. 2, B; 3, A). 

 This migration is all the more remarkable as there seems to be no 

 purely mechanical reason why it should take place. The progeny of 



1) Conn, H. W., Life History of Thalassema. Stud. Biol. Lab. 

 Johns Hopkins Univ., Vol. 3, 1886, No. 7. 



