DEVELOPMENT IN PLANARIANS. 2/1 



Blochmann, 1 who studied the tissues of cestodes by means of 

 the methylen-blue and silver impregnation methods, describes as 

 " Sommer-Landois cells " large cells lying below the surface mus- 

 culature and sending out branches connected on the one hand 

 with the fibers of the surface musculature and on the other hand 

 extending into the peripheral nerve plexus. The muscle fibers he 



FIG. 6. Sections illustrating the development of the musculature. Magnification, 

 540 diameters. 



a. Myoblasts in an embryo in which the pharyngeal anlage is being formed. 



b. Developing circular musculature at a later stage. 



c. Developing parenchymal muscle cell. 



d. e. Small nuclei lying between the muscle fibers of an adult individual. 

 _/. Large cell occupying a corresponding position. 



g. Developing circular musculature during regeneration. 



considers outgrowths of processes of these cells. In Planaria 

 maculata large cells with vesicular nuclei, resembling in size and 

 form the myoblasts of the embryo, may be seen lying below the 

 surface musculature. Whether or not these represent Sommer- 

 Landois cells which have migrated from the syncytial musculature 

 after the formation of the latter cannot be decided by methods 

 which I have employed. Monti has described cells of the Som- 

 mer-Landois type in planarians. 2 



It is uncertain whether the musculature of the pharynx arises 

 from the cells potentially differentiated at the time of the forma- 

 tion of the pharyngeal anlage. 



Of all the tissues the nervous tissues are the most difficult to 

 follow in these animals. Only by special technical methods can 

 this be satisfactorily done and yet the planarian tissues seem very 

 refractory to the ordinary impregnation methods. Blochmann 

 (pp. cit.) used these successfully on cestodes and gives an inter- 



1 " Ueber freie Nervenendigungen und Sinneszellen der Bandwiirmern," Biolo- 

 gisches Centralblatt, XV., 14, 1895. 



2 Archives Italiennes de Biologic, Vol. XXVII., p. 15, 1897. 



