The Development of Phascolosoma. HI 



and becomes attached to a group of ectoderm cells, which lie near 

 the middle of the dorsal side of the trunk [Fig. 62, 63, 89). This 

 duster of cells, which is to give rise to the proctodaeum, becomes 

 slightly invaginated during the period of metamorphosis (Fig. 63). 

 No lumen appears in the archenteron until a still later period (5th 

 or 6th day), when the yolk within the endoderm cells has in large 

 measure disappeared. The yolk granules of the archenteron are 

 distinctly oner than those within the prototroch cells. 



8. Transformation into the Larva (48—60 hours). 



At the end of the second day the elongated top-shaped trocho- 

 phore casts off its yolk membrane, and suddenly assumes a cylin- 

 drical form (Fig. 51—53, 62, 65, 68), with a I'ing-shaped swelling 

 at the anterior end, immediately behind the apical plate. This 

 annular projection is the prototroch, the cells of which disappear 

 rapidly from view during the next few hours. A constriction of 

 the body, due to the contraction of the posttrochal muscle, or middle 

 sphincter, occurs immediately behind the prototroch, giving the young 

 larva a somewhat "wasp - waisted" appearance (Fig. 52, 62, 68). 

 After the disappearance of the prototroch, the larva becomes cylin- 

 drical. Fig. 49, 52, 53, drawn from the same lot of living specimens, 

 show this change of form and the disappearance of the prototroch, 

 as well as the replacement of the zona radiata by the cuticula. The 

 most striking features of the metamorphosis, as viewed from the 

 surface, are the shedding of the yolk membrane and the development 

 of muscular activity, particularly the beginning of the introversion 

 of the head. 



Shedding of the Yolk Membrane (Fig. 51, 68). At the 

 age of about forty-eight hours the elongated trochophores, as already 

 pointed out, show a marked tendency to sink to the bottom of the 

 aquarium, where they continue to twirl during the metamorphosis. 

 The egg membrane now gives way to the pressure that is exerted 

 upon it by the rapidly growing trunk, and is rent open, usually 

 along the line of attachment of the postoral circlet of cilia in Ph. 

 vulgare. In most cases the posterior part of the membrane is shed 

 at once, and the postoral cilia of this species soon free themselves 

 by slipping through the pores of the loosely attached membrane or 

 of its pieces. The delicate adorai cilia of the prototroch in many 

 individuals appear to be destroyed during the process, but in sections 



