644 L. E. Cary. 



from -Lückeuräume" between the cells of the '-primitive parenchyma" : 

 "Zunächst sind Trichter sowohl wie Gefäß nicht aus hohlen Zellen 

 zusammeng-esetzt, wie ich in einer früheren Arbeit anzunehmen 

 geneigt war, sondern es sind Lückenräume zwischen den Zellen." 

 These spaces run together in a regular manner and by this 

 process there are formed two continuous canals running longitudinally 

 through the body of the embryo. 



As opposed to the view of the "Lacuna Development" of this 

 system stand the observations of Bugge (1902), for the cercaria, 

 and of EossBACH (1906) for the redia. These investigators agree 

 that in the development of both these forms the water vascular 

 canals are formed from primordia which consist of two cell 

 strands, one on each side of the body. The cytoplasm belonging to 

 these cells becomes arranged on one side of the nuclei to form a 

 •'homogenen einheitlichen Plasmaschlauch". Within this protoplasmic 

 strand there arises, during the later development, a fine lumen which 

 forms the water vascular canal. 



My observations on the formation of the water vascular system 

 in DiplocUscus are strictly in accord with the last mentioned view. 

 In the young embryos (Fig. 56) certain cells on each side of the 

 endoderm mass become arranged in longitudinal strands, on the 

 outer side of which there is a well defined layer of cytoplasm which 

 in sections appears as a clear thread. The lumen of the canal first 

 appears in this clear strand close to the nuclei. As the lumen of 

 the canal increases in diameter, during the later development, the 

 nuclei of some of the cells through which it was formed, come to 

 lie in the lumen of the canal. These nuclei apparently degenerate 

 and are forced out to the exterior. 



The terminal portion of this system — the excretory bladder and 

 the canal leading from it to the dorsal pore — develops simultaneously 

 with the lateral trunks. The bladder first appears as a circum- 

 scribed mass of cells, which are cut off from their neighbors by a 

 wall of thin cells. Within this mass of cells there soon appears a 

 small cavity which rapidly increases in size through the degene- 

 ration of the cells within the primordium. The terminal canal is 

 formed from a continuation of this same cell mass which extends 

 from the body of the primordium to the dorsal wall of the body of 

 the cercaria. 



The flame cells are formed from meristem cells along the course 

 of the canals. At the beginning of the process these cells may be 



