IV. i INITIAL STRUCTURE OF THE GERM 213 



9. CHAETOPODA AND MOLLUSCA. 



In the Chaetopoda and Mollusca (except the Cephalopods) it is 

 possible to trace back almost every organ of the body to some parti- 

 cular cell, or group of cells, of the segmenting ovum, to describe, 

 in fact, its lineage in terms of individual cells. Specific materials 

 for the formation of the various parts seem to be sundered from 

 one another by the process of cleavage, which thus presents the 

 characters of a ' Mosaic ' work in an exceptional degree. To 

 what extent such a preformed structure does in reality exist 

 experiment alone can decide, and the answer given by experiment, 

 so far, at least, as the nucleus is concerned, is in the negative. 



By means of pressure Wilson succeeded in preventing in Nereis 

 the normal formation of the first quartette of micromeres ; instead, 

 the egg divided into a flat plate of eight equal cells. The pressure 

 was then removed. All eight cells formed micromeres, and later 

 on eight instead of the usual four macromeres were found in the 

 Trochophore larva. Four of these necessarily contained nuclei 

 which would normally be placed in cells of the prototroch, and 

 yet the larva was normal. In this case, then, the causes of 

 differentiation cannot lie in the nucleus; they must be sought 

 for, if anywhere, in the cytoplasm. 



In the eggs of these forms segmentation is holoblastic and of 

 the spiral type, with the cleavages alternately dexiotropic and 

 laeotropic, and it is easy not only to determine the lineage of 

 each organ, but to compare the origin and the destiny of indivi- 

 dual cells in a large series of forms. The first, second, and third 

 quartettes of micromeres are usually destined for ectoderm 

 formation, the second quartette cell in the D quadrant, 2 d, being 

 the first somatoblast from which the ectodermal structures of the 

 trunk are derived. The ciliated ring or prototroch is formed, in 

 most cases, from certain derivatives of the first quartette, namely 

 1 a 2 1 d 2, the primary trochoblasts, each of which divides 

 into a group of four; but it may be reinforced by secondary 

 trochoblasts from the same or the second quartette. The second 

 and third quartettes may provide larval iresoderm or mesenchyme. 

 The trunk mesoderm is usually derived from 4 d. The remaining 



