THE DIFFERENTIATION OF THE MESODERM-RUDIMENT, ETC. 211 



it is found to arise by the concentration of mesoderm-cells at the 

 base of the foot. 



The development of the mesoderm and of the parts connected with 

 it has recently been specially studied by v. Erlangek in Paludina 

 and Bythinia. As v. Erlangeb found that these organs developed 

 here in the same way as in the Lamellibranchs, and, since the 

 investigations of other zoologists which were less comprehensive led 

 to less satisfactory results, we shall here follow principally the state- 

 ments of this author. 



The two mesoderm-sacs, above mentioned, approach each other 

 ami come to lie ventrallv between the archenteron and the ectoderm. 



iii€a. 



TiWs: 



Fig. '-'7. —Transverse section through the pericardial region of an embryo of Paludina 

 viviparasX tin- stage depicted in Fig. 59 II. /> (after v. Erlanger). /, liver ; Ih, 

 body-cavity; m, stomach; ines, mesodermal tissue; mf, mantle-fold; mli, mantle- 

 cavity; ", rudiment of definitive, n', of abortive kidney; /»', //"'. rudiments of 

 efferent ducts of the same ; />. pericardium : s, shell. 



where they fuse. Occasionally, in later stages, a septum is retained 

 as an indication of the former partition-wall (Fig. 98 J, sp). In the 

 further course of development, the right half of the sac grows much 

 more vigorously than the left, and the whole sac extends dorsally to 

 thr right side (Figs. 59 A, and 97). Differentiation now sets in, the 

 walls of the two later ventral angles of the sac becoming thickened 

 and subsequently forming distinct outgrowths (Fig. 97, // and //'). 

 These outgrowths, according to Erlangek, are the rudiments of the 

 definitive kidneys which are consequently, like the pericardial sacs. 

 paired on their first appearance. The left rudiment soon disappears, 



