162 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
2. Polar cells 2 ALM MRC TMA, «wa. cistsag oily’ Gi 06 Leake 
3. Wleavage 2c.C Eager en tee tote oe ea 
4. Double nuclei Pa Pale i Age On PCED 
Addendunnawiic.  sutey sowie eins 4. ee 
VI. ° Bibliography. y b.). sekyes: mae ee ees Sepa: (eae) hs ee 
Explanation of plates 
I. Introduction. 
The literature on the embryology of the Coelenterata, and more 
particularly 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. ‘lhe 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 Zodlogical Laboratories of Harvard University. 
To Dr. F. B. Sumner, Director of the U. 5S. 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. Mar 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. 
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