1 8 Edmund B. Wilson. 



zones visible in the living egg of Myzostoma, as observed by 

 Driesch ('96), where the lower polar area is represented by a 

 green substance, the upper one by a reddish material, and the 

 pigment zone of Dentalium by a zone of clear protoplasm. It 

 is important not to confuse the above-described distribution of 

 white and pigmented material with that of protoplasm and deu- 

 toplasm. As shown on a preceding page the upper white area is 

 not, like the lower one, free from yolk; and in point of fact all 

 the cells contain a large amount of yolk. The pigment-pattern is 

 only a visible expression in the living object of a distribution of 

 specific materials that can only in part be distinguished in sections. 

 We may now briefly consider the main outlines of the larval 

 development. In warm weather the embryos become ciliated at 

 about the ninth or tenth hour, and at the end of twenty-four hours 

 are well developed trochophores that swim very actively at the 

 surface, progressing in a spiral curve and rotating from right to 

 left as seen from the side. At this period (Fig. 29) the body is 

 of a blunt spindle-shape, encircled at the equator by a very broad 

 prototroch composed of three principal rows of large trocho- 

 blasts which bear three corresponding rows of powerful cilia 

 completely encircling the body and leaving no dorsal gap (as is 

 also the case in Patella). The pre-trochal and post-trochal re- 

 gions, while somewhat variable, are at this period nearly similar 

 in form and size, being roughly conical and rounded at the tip. 

 The pre-trochal region is wholly covered with very short vibra- 

 tile cilia and bears at its apex a very long and well-defined tuft 

 of flexible, but not vibratile, flagelliform sensory hairs. In total 

 preparations, or in longitudinal sections, it may be seen with 

 great clearness that the apical tuft is borne upon a large and 

 definitely circumscribed apical thickening or plate, sharply marked 

 off from the surrounding cells. The post-trochal region is not 

 ciliated, but bears at its posterior extremity a small bunch of 

 sensory hairs, which differ from those of the apical tuft in being 

 quite stiff, and radiating from the common point of attachment. 

 The ahmentary canal at this period forms a closed sac divided 

 into two chambers, into one of which at a slightly later period 

 opens the mouth, formed immediately below the prototroch, but 



