1904.] NATURAL SCIENCES OF PHILADELPHIA. 709 



fact I have not been able to trace with certainty the dorsal posterior 

 limits of the first quartette. On the ventral side the boundary of 

 the first quartette is indicated in fig. 55 by the most anterior row of 

 small cells, and curves forward in a semicircle in front of the blastopore. 

 The position and general outline of the first quartette are sufficienth- 

 indicated in text fig. IV, E. It covers like a cap the anterior end of the 

 embryo and corresponds to the umbrella of the trochophore. 



At the time of the appearance of the stomodseum the ectoderm at the 

 anterior end of the embryo is seen (fig. 58) to consist of high columnar 

 ■cells, while the ectoderm cells covering the rest of the embryo — except 

 at the posterior end, where are the still large remnants of the X cells — 

 are cubical or flattened. The centre of this thickened area is not pre- 

 cisely the anterior pole of the larva, but slightly dorsal to that point, at 

 which place the descendants of the rosette were last recognized. This 

 thickening is the rudiment of the brain of the adult. At a later stage 

 the cells of the thickened area have multiplied and become so closely 

 crowded together that their outlines are barely distinguishable. In fig. 

 ■59 the brain rudiment has increased both in thickness and extent, and 

 has at the same time moved somewhat dorsad owing to the rapid 

 growth of the ventral plate. In fig. 58 the thickened area represents, 

 as comparison wdth text fig. IV. E, will show, less than half of the first 

 quartette; in fig. 59 it has increased to nearly twice its former area. 

 Fig. 60 shows the brain in sagittal section at a slightly later stage. On 

 the dorsal side, just behind the brain, is a deep furrow which in the 

 adult separates the head from the trunk. The lateral extent of the 

 brain is shown in fig. 61, a horizontal section of a stage more advanced 

 than that shown in fig. 60. In this embryo the brain already shows 

 signs of its bilobed character. From these figures it can be seen that 

 nearly the whole of the first quartette is involved in the formation of the 

 brain, if we assume that the second preoral ciliated band of the adult 

 arises from the trochoblasts, as is probably the case, since the brain 

 rudiment is limited posteriorly by the constriction which separates 

 the head and trunk; while the second preoral ciliated band of the adult 

 (prototroch) appears on the elevation of the head just anterior to this 

 constriction. 



The cell origin of the cerebral ganglia among the mollusks has been 

 very fully described by several recent investigators (Conklin, 1897; 

 Holmes, 1900; Meisenheimer, 1901; Robert, 1903); among the anne- 

 lids by von Wistinghausen (1891) and Ijy Wilson (1892) for Nereis, 

 by Mead (1897) for Amphitrite, by Eisig (1898) for Capitella. Wilson 

 has shown that A'on Wistinghausen's account was incomplete and 



