INVEETEBEATA 



CHAP. 



312 



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

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

 earhest stages of the formation of the mesoderm in Faludina are 

 precisely similar to later stages in the development of the mesoderm 

 in Phi/sa, and other forms, whose cell -lineage has_ been worked 

 out in the greatest detail. 



In all these cases the origin of the adult 



r^*:^ 





l.per 



-^Eaisiws'*' 



sept 



Fig. 247. — Two stages in tLe forjiiation of tlie pericardium ot Paludina mvipara. 

 (After Erlauger.) 



A, horizontal section through visceral hump, of stage in which the two inesodermic 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 mesodermic hands have fused in the middle line to form the septum 

 separatiug the right and left pericardial .sacs, g, gut ; l.'p&r, left pericardial sac ; r.rper, 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 



