No. 3-] THE CELL-LINEAGE OF NEREIS. 399 



brane, over a small area from which the cilia are immediately 

 put forth. At this stage the embryo recalls the larva of Sipun- 

 adus at the time of the amnion-formation (see Hatschek, No. 

 11). The space surrounding the apical tuft is, however, soon 

 obliterated, and the larva again becomes spherical and closely 

 surrounded by the membrane. It seems possible, nevertheless, 

 that this peculiar process may give the key to an explanation 

 of the origin of the amnion in other forms. 



The prototrochal cilia are put forth from the large cells alone 

 and not from the adjoining smaller cells. Nevertheless, the 

 girdle of large cells is accompanied by a second girdle of smaller 

 cells lying just above it. It is only in later stages that this 

 auxiliary row becomes clearly defined (see Figs. ']'], j"^, 82), 

 when the cells are found to correspond accurately in number 

 and position with the large cells. I much regret my failure to 

 determine their origin, and this is one of the greatest gaps in my 

 work. Their precise correspondence in position with the large 

 cells makes it seem probable that they are derived from the 

 latter by horizontal divisions ; but, except in the case of rt^-^-^, 

 ^1.1.2^ ^1.1.2^ g^j^(-| ^1.1.2^ J have never seen the least evidence of such 

 a division, although a large number of embryos were examined 

 to determine the point. This may perhaps be taken as indirect 

 evidence that the auxiliary cells are differentiated from the cells 

 of the upper hemisphere — i.e. from the products of the inter- 

 mediate girdle-cells and from the four arising by the horizontal 

 divisions of a)--^-'^, etc. This conclusion is, however, of too great 

 importance to be accepted on purely negative evidence and 

 must be tested by future research. The origin of the ring- 

 nerve and of the ring-muscle I have not yet studied. 



V. Bilateral Period of Cleavage. 



With the completion of the prototroch, the cleavage loses 

 every trace of the spiral symmetry of earlier stages, and becomes 

 strictly bilateral. The median plane, with respect to which the 

 bilaterality manifests itself, is the plane of the second cleavage, 

 which finally persists as the median plane of the adult body. 

 Hence the appearance of the bilateral divisions is the first defi- 

 nite step in the long series of changes by which the spirally 

 symmetrical embryo is converted into the bilateral adult. 



