2 LILLIE. [Vol. X, 



PAGE 



III. Gastrula to Glochidium 46 



a. Gastrula to Young Larva 46 



1. Ectodermal Organs 46 



2. Mesoblastic Organs 54 



3. Entodermal Organs 58 



4. Summary 58 



b. Transformation of the Young Larva into the Glochidium 60 



C. Axial Relations 71 



IV. Appendix 76 



The work recorded in the following pages was begun in the 

 summer of 1891 at the Wood's Holl Marine Biological Labora- 

 tory, at the suggestion of Professor C. O. Whitman. It has 

 been pursued since, subject to more or less interruption, at 

 Clark University and the University of Chicago. In each 

 laboratory I profited by the instruction and advice of Professor 

 Whitman. It gives me great pleasure to acknowledge in this 

 place my deep indebtedness to him. 



I. Introduction, 



The Unionidae are in many ways extremely favorable objects 

 for embryological research, and in consequence have often been 

 the subject of more or less careful investigation. The great 

 ease of obtaining an almost limitless supply of material, the 

 comparatively large size of the eggs, and the transparency of 

 many of the larval stages make them favorable for embry- 

 ological work. On the other hand, the early stages of develop- 

 ment offer considerable technical difficulties to the student. 

 Moreover, the extremely specialized character of the develop- 

 ment seems to promise but little of general morphological 

 interest. 



The history of recorded observations on the embryology of 

 the Unionidae is a long story; and as it has been rehearsed 

 more than once by other writers on the subject (Rabl No. 25, 

 Flemming No. 13), I need only say enough here to make my 

 point of departure clear. A list of the early literature on the 

 subject will be found at the end of this paper. The question 



