312 



CHAP. 



confidently say that, whatever may be the true state of affairs, 

 Tonniges is most certainly wrong. For what he figures as the 

 earliest stages of the formation of the mesoderm in Paludina are 

 precisely similar to later stages in the development of the mesoderm 

 in Physa, and other forms, whose cell -lineage has been worked 

 out in the greatest detail. In all these cases the origin of the adult 



FIG. 247. Two stages in the formation of the pericardium of Paludina vimpara. 

 (After Erlanger.) 



A, horizontal section through visceral hump, of stage in which the two mesodermic band sare still 

 separate a rudiment of the pericardium has appeared in each. B, horizontal section through visceral 

 hump of later stage, in which the two mesoderinic bands have fused in the middle line to form the septum 

 separating the right and left perieardial sacs, g, gut ; l.per, left pericardial sac ; r.per, right pericardial 

 sacs ; sept, septum formed by the opposed walls of the pericardial sacs ; tr, trabecula of cells crossing 

 right pericardial sacs. 



mesoderm has been traced to the cells of the fourth quartette, which, 

 as in Annelida, are part of the endoderm. 



The later products of the division of the mother cells of the 

 mesoderm, it is true, often come into such close contact with the 

 ectoderm that, if one had not a complete series of the earlier stages 

 to examine, one would believe that there was demonstrative proof 

 that the mesoderm was derived from the ectoderm ; and indeed this 

 very mistake has been made by other German workers in the case of 

 other Mollusca (Meisenheimer, 1898, 1901, and Harms, 1909). The 



