EMBRYONIC AND REGENERATIVE DEVELOPMENT 



IN PLANARIANS. 



CHARLES RUSSELL BARDEEN, 

 MARINE BIOLOGICAL LABORATORY, WOODS HOLL, MASS. 



The term "regenerative development" may be applied to the 

 formation of a new from a part of a preexisting individual 

 whereby certain structures are carried over essentially unchanged 

 from parent to daughter. In the following paper regenerative 

 and embryonic development in Plauaria maculata are compared. 



EMBRYONIC DEVELOPMENT. 



The classical account of the embryonic development of plan- 

 arians is given in the excellent paper of lijima, published nearly 

 twenty years ago. 1 lijima studied more especially the develop- 

 ment of Dendroccelum lactcmn. The development of PI an aria 

 maculata seems to be essentially similar. With the early em- 

 bryonic stages down to the formation of the " embryo-pharynx >; 

 we are not at present concerned. It is the transformation of the 

 spherical embryo, filled with yolk cells, into a worm of normal 

 structure and proportions that is of special interest from the 

 standpoint of regenerative development. 



At the period when the imbibition of yolk cells begins to be 

 active the planarian embryo is spherical in form. It is surrounded 

 externally by a well-marked membrane formed of greatly dis- 

 tended, flat epithelial cells, shown in section in Fig. 2, A. Be- 

 tween this membrane and the yolk cells which fill the central 

 cavity is a reticular substance in which are scattered large cells 

 with nuclei containing one or more nucleoli. These cells are 

 represented by the larger bodies in the body-wall shown in Fig. 

 2, A. One of them is shown, in Fig. 4, a. The nucleus is im- 

 mediately surrounded by a granular mass of protoplasm. The 

 reticular substance of the body wall probably represents the 



1 lijima : "Unters. uber die Bau u. die Entwicklungsgesch. d. Siisswasser-Dendro- 

 ccelen (Tricladen)," Zeitsch rift f. Wissenschaf (lithe Zoologie, Vol. XL., 359, 1884. 



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