No. 3-1 • THE CELL-LINEAGE OF NEREIS. 393 



close of this period marks also the complete differentiation, not 

 only of the germ-layers, but also of many of the protoblasts 

 from which the adult organs arise. The segregation of the 

 embryonic material is in fact so nearly completed, that this last 

 spiral stage may be taken as a new point of departure. The 

 embryo (Fig. 29) now consists of thirty-eight blastomeres, as 

 follows {cf. also the diagram, p. 440) : 



4. The macromeres A, B, C, D, or entomeres = Entoblast. 



4. The first group of micromeres, d^, b^, c^, d'^, 



^M.i ^1.1.2 ^1.1.1 31.1.2 

 8. The products of the trochoblasts, , ,/,,„'„,,'„, ,' 



4, The four intermediate girdle-cells, a^•'^, b^'^, c^-^, d^-'^, 



4. The rosette-cells, a^-^, b^-^, c^-^, d'^-'^, }- Ectomeres = Ectoblast. 



3. The three smaller secondary micromeres, (Z^-i, b--i^ ^21^ 



3. The stomatoblasts, d^-'^, b'^-'^, c'^-^, 



3. The first somatoblast (X) and its progeny (jc^, jc^), 



4. The four tertiary micromeres (a^, b^, c^, d^), 

 I. The second somatoblast or mesomere . = Mesoblast. 



38 



Each of these ten categories of blastomeres might be taken 

 as the starting-point for a separate description. Practically, 

 however, it will be useful to pursue the general development 

 somewhat further before turning to the history of the individual 

 cytogenies. The embryo may now be termed a " gastrula," in 

 so far as it consists of an "inner" and an "outer" layer. If I 

 employ this term, it is, however, solely for the sake of conven- 

 ience. The embryo is "two-layered" only in a conventional 

 sense. The "outer layer" is a mes-ectoblast in which the two 

 constituents are completely separate. The origin of the meso- 

 blast in Nereis is ectoblastic (because it forms a part of the 

 "outer layer"), or entoblastic (because it arises from one of 

 the four entomeres), or neither (because it forms the lip of the 

 "blastopore"), according to the reader's preference. 



IV. Transition to the Bilateral Period. 



As far as the development of the permanent organs is con- 

 cerned, the transition from the spiral to the bilateral type of 

 development is remarkably abrupt. It is only in the peculiar 

 changes involved in the formation of a larval organ, the proto- 

 troch, that the spiral form of division overlaps the bilateral 



