The Development of Phascolosoma. ]^25 



great that it is not surprising that he overlooked the fact that his 

 "Blutkörperchen" (masses of yolk granules) are derived from the 

 cells of the prototroch, rather than from one of the large cells of 

 the "vegetative" half of the egg, which become covered by the cells 

 of the prototroch. 



Cilia appear early, and later form a continuous broad postoral 

 band. A preoral circlet of delicate cilia was observed in the older 

 embryos, and long cilia, which he regarded as abnormal, appear on 

 the apical plate. These later become transformed into (5—8) short 

 bristles, which he believed to be normal, since he found them to 

 be regularly present in the younger embryos. In this connection it 

 should be noted that, in the forms that I have studied, the long 

 llagella are regularly broken off into the form of short processes, as 

 soon as the introversion of the head begins. 



Selenka observed the appearance of pigment spots in the apical 

 plate and the positive heliotropism of the embryo. He found that 

 the nerve cord arises as a ventral thickening of the ectoderm, which 

 he described as continued also in front of the mouth into a conical 

 and hollow prostomium (Kopflappen). In Ph. gouUii and Bi. vulgare 

 I have found that the prostomium contains the solid rudiment of 

 the supraoesophageal ganglion, in the region where Selenka's figure 

 of an optical section shows a cavity. Selenka was evidently in 

 error in regard to this point, as well as in believing that the yolk 

 membrane becomes transformed into the cuticula, which I have 

 already discussed at length (p. 111). The paired lateral bristles, 

 which he described, I have shown furthermore to be peculiar to the 

 Villefranche form which he studied. 



On the fourth day, in Selenka's larva, a circlet of 6—9 hook- 

 like setae were formed below the mouth opening and immediately 

 in front of the postoral band of cilia. It is stated that these 

 increase in number, and become the anterior circlet of hooks in the 

 definitive armature of the introvert. The well-rounded blunt extre- 

 mities of these hooks in Selenka's figure, and their position, suggest 

 to me the remains of the egg membrane, which often cling in shreds 

 in this particular region for a considerable time after the rupture 

 of the yolk membrane. Since only momentary glimpses of the head 

 region can be had during the brief intervals of elongation, this 

 mistake would be at least possible. It must be acknowledged, 

 however, that the discovery by him of an older larva, probably of 



