"VI. — The Maturation, Fertilization and Early Develop- 

 ment OF THE Planarians. By Willard G. Van Name. 



Illustrated by plates XXXVI to XLI inclusive. 



The eggs of some species of planarians, particularly those of some 

 of the polyclads, offer many advantages for the study of the processes 

 of maturation and fertilization. The ease with which many of these 

 animals may be kept in captivity, and the abundance of eggs which 

 they produce when kept under the right conditions, make the mate- 

 rial easy to obtain if the adult animals can be secured at the proper 

 season, while the fact that the eggs are fertilized within the body of 

 the adult jsrotects them from the unnatural conditions which so often 

 produce abnormalities and pathological changes in eggs artificially 

 fertilized. 



There are, however, dilficulties in the way of preserving these eggs 

 which make good preparations not easy to obtain. The membrane 

 enclosing the egg is very thick and is penetrated by preserving fluids 

 with difliculty. In addition to this they are in many cases thickly 

 coated with mucus, which is coagulated by the reagent and offers 

 much resistance to its penetration. These envelopes of the eggs are 

 also apt to cause trouble in imbedding and in cutting sections. 



The consequence has been that although the eggs of a number of 

 species of planarians have been studied by different investigators, 

 there is uncertainty and disagreement in regard to many points 

 which can hardly be due to essential differences between the species 

 studied, since with one exception they were all polyclads, as are also 

 the two which ai'e the subject of the present paper. 



Mitotic figures were observed in the eggs of planarians by Selenka 

 (21) and Lang (18) many years ago and a series of short papers was 

 published by Van der Stricht (22 to 27 inclusive) on the egg of 

 Thysanozoon Urocchi, but the principal contributions to our knowl- 

 edge of the subject have appeared within the last two or three years. 

 These are the work of Klinckowstrom (14) on the egg of Prosthe- 

 cerceiis vittatus, the two papers of Francotte (3, 4) describing and 

 illustrating those of the same and several other polyclads, and a later 

 work of Van der Stricht (28) on the egg of Thysanozoon. The 

 investigations of the two last mentioned writers especially have done 

 a great deal to clear up the doubtful points and bring our ideas of 



