No. 3-] THE TAIL IN LUMBRICULUS. 333 



berg says that a rational classification of tissues can be made 

 only from a physiological standpoint. " Alle embryologischen 

 Erscheinungen laufen in — meist sehr weit zuruckliegende — 

 physiologische Zustande aus. So wie die Sachen liegen, verstehe 

 ich nicht, auf welche Weise die zahlreichen und heterogenen 

 Organe, welche konventionell vom mittleren Keimblatt abgeleitet 

 werden, aus einer einheitlichen indifferenten Anlage entstehen 

 konnten." 



It is doubtless in harmony with the view that there is no 

 middle germ layer to consider that the tissues commonly termed 

 mesoderm have collectively no worth independently of the two 

 primary germ layers. Although from this point of view the 

 segregation of mesoblast into the primary mesoblasts may be 

 regarded as analogous to the segregation of the material of the 

 central nervous system into neuroblasts, yet the difference in 

 the regeneration of the nervous system and of the mesoderm — 

 the recurrence to the three-layered condition of the embryo — 

 seems to me to indicate a marked difference in the rank of 

 the two tissues. 



VII. Peritoneum and Neoblasts. 



It seems probable that the peritoneum, as the least differen- 

 tiated of the mesoblastic tissues and in some of the AnneUds 

 very little changed from the embryonic condition, contains the 

 mesoblastic elements. This is in harmony with the theory of 

 Weismann (24) that the complexity of protoplasm is in propor- 

 tion to its non-differentiation. 



This view of the condition of the peritoneum is the one most 

 generally accepted. Von Kennel says it is in Ctenodrilus "als 

 undifferenzirtes Mesodermgewebe zu betrachten " ; and Rohde 

 (18), for the Limicolse in general, " Dieses Gewebe ist in engsten 

 Zusammenhang mit der Bildung der Muskulatur zu bringen," 

 and "ist als Bildungsgewebe der Muskeln zu betrachten." 



The other view is that of Kleinenberg, who considers the 

 peritoneum of Lopadorhynchus to be transformed muscle tissue 

 and hence a tertiary structure, as the muscles are a secondary 

 formation from the ectoderm. 



Billow does not give the details of the formation of the 

 muscles in the regeneration of Lumbriculus and makes no 



