514 PROCEEDINGS OF THE ACADEMY OF [DeC, 



THE EARLY DEVELOPMENT OF A POLYCLAD, PLANOCERA INQUILINA Wh. 



by frank m. surface. 



Contents. 



Introduction. 



Materials and Methods. 



Nomenclature. 



The Li\'ing Eggs. 



General Account of the Embryology. 



The Unsegmented Egg. 



The First Cleavage. 



The Second Cleavage. 



Segregation of the Ectoblast. 



TMrd Cleavage. 



Fourth Cleavage. 



Fifth Cleavage. 

 Formation of the Fourth Qiiartet. 

 History of the First Quartet. 



The Apical Cells. 



Further Divisions of the First Quartet. 

 History of tlie Second and Third Quartets. 

 History of the Fourth Quartet. 



The Mesoblast. 



The Entoblast. 



Discussion. 

 Summary. 

 Literature List. 

 Explanation of Plates. 

 Tables of Cell Lineage. 



It gives me great pleasure to express my deep indebtedness to Prof. 

 E. G. Conklin for his many suggestions and kind encouragement 

 throughout this and other work. I also wish to acknowledge the 

 many favors I have received from the University of Pennsylvania by 

 which this work was made possible. My thanks are also tendered to 

 the donor of the University of Pennsylvania room at the Marine 

 Biological Laboratory at Woods Holl during the summer of 1906. 



Introduction. 



Our knowledge of the development of the polyclad worms is based 

 chiefly on the admirable works of Arnold Lang. In his extensive 

 monograph of this class of the Turbellaria, published in 1884, Lang has 

 not only summarized the results of all previous investigators, but has 

 himself added very materially to our knowledge of their embryology. 



