162 bulletin: museum of comparative zoology. 



2. Polar cells 206 



3. Cleavage 206 



4. Double nuclei 206 



Addendum 207 



VI. Bibliography 208 



Explanation of plates 



I. Introduction. 



The literature on the embryology of the Coelenterata, and more 

 ])articularly on that of Hydromedusae, contains very few articles 

 dealing with cytologic details of early stages, though there are a con- 

 siderable number dealing with the more general questions of cell 

 division, later development, etc. Cytologists seem largely to have 

 neglected these forms, which may be of considerable importance in a 

 thorough understanding of cell structure and development. The few 

 papers which deal with the early history of the egg of Hydromedusae 

 show numerous gaps, and also suggest many points of difference 

 between the Hydromedusae and other groups of invertebrates. The 

 accounts of the history of the germinative vesicle during the periods 

 of maturation and fertilization have been especially unsatisfactory 

 and conflicting. It has therefore seemed worth while to study again 

 with care these stages in the same forms as those upon which some of 

 the earlier work was done. 



The problem was suggested to me by my father, C. W. Hargitt, 

 several years ago, and the work was started during 1906 at Syracuse 

 University. The summers of 1907 and 1908 were spent upon the sub- 

 ject at the Laboratory of the United States Fisheries Bureau at Woods 

 Hole, and during the past two years the work has been carried to 

 completion in the Zoological Laboratories of Harvard University. 



To Dr. F. B. Sumner, Director of the U. S. Fisheries Laboratory 

 at Woods Hole, I am indebted for facilities in collecting material, and 

 for laboratory privileges. To Dr. C. W. Hargitt I owe much in 

 suggestion and enthusiastic interest, especially during the early part 

 of the work; and to Dr. E. L. INIark I am indebted for constant and 

 helpful criticism and supervision during the past two years. To all 

 of these it is a pleasure to express my appreciation of their aid and 

 interest. I have also had the privilege of examining the preparations 

 and drawings of the early stages of development of the eggs of Hydrac- 

 tinia and Pennaria which will form the basis of a forthcoming paper by 

 C. W. Hargitt and W. M. Smallwood, to whom my thanks are due 

 for this opportunity. 



