CELL-LIXEAGE. 



The Relations between Mesoblast and Entoblast in 

 Annelids and Mollusks. 



In Xcreis, as in the typical development of other annelids and 

 of gasteropods and lamellibranchs, the mesoblast-bands are de- 

 rived from the posterior cell of the fourth quartet of '' micro- 

 meres."^ This cell, now generally known as the second soniato- 

 blast, divides into two symmetrical halves which have been usu- 

 ally designated as the ''primary mesoblasts ;" and from them, by 

 a series of slightly unequal successive divisions, arise the meso- 

 blast-bands which extend forward in the cleavage -cavity at the 

 sides of the embryo. Before giving rise to the mesoblast-bands, 

 however, the ''primaiy mesoblasts" bud forth the small cells 

 already referred to, at or near the surface directly behind the 

 two posterior macromeres '' C " and " D." At least six, and 

 probably not less than ten, of these cells are formed, the primaiy 

 mesoblasts meanwhile sinking below the surface and becoming 

 quite covered by ectoblast-cells which advance from the sides 

 and from behind. The small cells first formed lie at the surface, 

 wedged in between the "primary mesoblasts" and the macro- 

 meres (Fig. I, D, c ; Fig. 2, B^. Those formed later lie 

 below the surface, owing to a change in the plane of division 

 (Fig. 3, A). The small cells, which are very conspicuous in 

 sections by reason of their intensely chromatic, closely reticu- 

 lated nuclei, thus become arranged in a thin plate extending 

 inwards from the surface between the primary mesoblasts and 

 the two posterior macromeres (Fig. 3, B). After the formation 

 of the small cells the divisions of the primary mesoblasts sud- 

 denly change both in form and direction, the plane of division 

 being now nearly or quite at right angles to the former (/. c, 

 approximately parallel to the sagittal plane of the embryo) and 

 the cells thus produced being nearly as large as the primary 



1 Nereis is somewhat exceptional in the fact that the other three cells of the 

 fourth quartet are suppressed. In Aricia, Polymnia, Spio, Pysgmobranchtis, Hy- 

 droides, Polygordhis (all of which I have examined), and in some others, the fourth 

 quartet, is complete, and in the first two forms named, a fifthquartet of (entoblastic) 

 micromeres is formed before the invagination (Cf. Fig. 2, AY 



