352 GEORGE A. BATES 



the ectoderm and somites. At first free, as seen in figure 9, it 

 later impinges upon the ectoderm, as shown in figures 1, 2 and 

 3, drawn from sections of an embryo of 5 mm., corresponding 

 closely to No. 19 of Scammon's Normentafeln. The growing tip 

 of the anlage of the duct extends along the ectoderm in close con- 

 tact with that layer, and in this stage has reached a length cor- 

 responding to that of three somites. Within the pronephric body 

 ;are numerous cells in mitosis and others are found also in the grow- 

 ing duct. In figure 1 the cavity of the somite (nephrocoele) has 

 begun to bend toward the anlage of the duct, marking the be- 

 ginning of the first pronephric tubule. The relation of the ecto- 

 derm to the anlage of the duct is very clearly shown in figure 3, 

 which is drawn under a one-twelfth oil immersion objective in 

 order to bring out definitely the existence of a limiting membrane 

 between the two layers. Figures 1, 2 and 3 represent consecu- 

 tive sections, figure 3 showing the growing tip and its relations to 

 the somites as well as to the ectodermic layer. They show the 

 extent of the growth of the anlage at this stage and demonstrate 

 its mesodermic origin and also the fact that the contribution of 

 cells from the ectoderm at this early period, when the anlage has 

 just come into contact with it, is improbable, because a hmiting 

 membrane is present between them. This will be made still more 

 convincingly manifest by sections to be presented in subsequent 

 figures. 



A cross-section of an embryo of 4 mm. (Normentafeln No. 16) 

 illustrates Balfour's description of the beginning of the develop- 

 ment of the pronephros from the middle plate. Van Wijhe, in 

 his discussion of Balfour's account, states that the soUd knob de- 

 iscribed by Balfour formed a part of the pronephros as demon- 

 strated by his series of sections. It will also be recalled that the 

 segmental duct of Balfour came into close contact with the ecto- 

 derm, and, but for its origin in the mesoderm, might be interpreted 

 as being derived from it (the ectoderm). Van Wijhe claimed that 

 this close contact was an actual fusion. It seems to the writer 

 that the sections represented in figures 1, 3, 4 and 5 settle the ques- 

 tion in favor of Balfour's interpretation. The demonstration of 

 the limiting membrane between the growing tip of the anlage of 



