STUDIES ON THE EMBRYOLOGY OF THE SIPUNCULID^. 449 



lively sluggish trochophore, the prototroch cells of which are heavily laden with yolk, 

 so that this species rises little from the bottom. In this form the postoral circlet of 

 cilia is only feebly developed. In P. vulgare there is less yolk, and the trochophore 

 is more active; it remains at the surface for a longer time than the American form, 

 and moves vigorously by means of a postoral circlet of cilia, even after the zona radiata 

 has been cast off. There is, however, much yolk both in the prototroch and in the 

 endoderm of this species, and consequently an epibolic gastrulation. In Sipunculus 

 nudus, on the other hand, there is very little yolk, and the trochophore is markedly 

 pelagic. It seems probable that the ancestors of Sipunculus possessed more yolk 

 than at present exists in the embryo, and that during the acquisition of the pelagic 

 habit the amount of yolk in the prototroch cells has been gradually reduced. The 

 large superficial extent of the serosal or prototroch cells, the fact that their substance 

 gradually wastes away, probably to be absorbed by the embryo, and the further con- 

 sideration that in Phascolosoma the corresponding structure is an organ of nutrition 

 lend favor to this assumption. 



If this supposition is true, it is readily understood how it has come about that 

 the non-ciliated cells of the body in Sipunculus lose their connection with the zona 

 radiata and sink beneath it, thus giving rise to amniotic cavities. This supposition 

 also offers an explanation of the invagination of the endoderm and the infolding of 

 the somatic plate; nor is it difficult to imagine how the prototroch cells under these 

 supposed conditions became spread out backward, slipping past the somatic and 

 endoderm plates till they covered the inside of the entire zona radiata and formed 

 the ciliated serosa. Accordingly the zone on which the postoral cilia were and still are 

 developed in Sipunculus lies no longer behind the prototroch, as is the case in Phas- 

 colosoma, but beneath it; and the appearance of cilia upon this zone is deferred until 

 the remnants of the surrounding prototroch or serosa are about to be cast off. 



VII. SUMMARY. 



A comparison of the development of Sipunculus, as described by Hatschek, 

 with that of Phascolosoma leads to these results: 



1. The following regions of the ectoderm of the trochophore are homologous 

 in the trochophore stages of Sipunculus and of Phascolosoma: 



(A) the apical region, with a characteristic definitive rosette at its centre; 



(B) the mid-dorsal cord, which extends backward from the apical region through 

 a break in the prototroch to the somatic plate; 



