1907.] NATURAL SCIENCES OF rillLADKLrill A. 83 



Hole during the summer of 1903; ami to Trof. L. B. Walton for extend- 

 ing to me the use of the biological laboratory at Kenyon College. I 

 wish especially to thank Prof. J. H. Comstock for the unfailing kindness 

 he has shown me at Cornell University. I wish also to thank my 

 friends, Dr. J. Percy Moore, of the University of Pennsylvania, and 

 Prof. A. D. ^lacliillivray, of Cornell University, for kindly advice and 

 criticism. 



II. Material and Technique. 



All of the material employed in the preparation of this paper was 

 obtained from the sea-water aquaria of the University of Pennsylvania. 

 This material was collected in three lots at three different periods. 

 The first lot was taken during the winter of 1901-2, and its preserva- 

 tion was only incidental to the collection of ova, which formed the 

 basis of a former paper by me (1904) on the early embryology of this 

 form. The individuals taken at this time .-were among the largest 

 obtained. The second lot was taken during the latter part of June and 

 the first few^ days of Juh^ 1903. These individuals averaged possibly 

 one-fourth to one-third smaller than those of the first lot. The third 

 lot was obtained during the winter of 1903-4. The individuals com- 

 prised in this lot are intermediate in size between those of the first 

 two lots, being much larger than those of the second lot, but scarcely 

 equalling those of the first. 



The methods of fixation employed were but three in number : 



(1) Kleinenberg's picro-sulphuric for one-half to three-quarters of 

 an hour, followed by a washing in 70 per cent, alcohol. This method 

 of fixation, which has given excellent results for many ova, including 

 those of this form, proved only an indifferent one for the mature in- 

 dividuals, and material fixed in this manner w-as unsuitable for the 

 study of any of the finer structures. It has, however, the advantage of 

 preserving the form and proportions of the object very well, and this 

 material was made use of largely as mounted entire. 



(2) A saturated aqueous solution of corrosive sublimate, containing 

 6 per cent, of glacial acetic acid, used boiling hot, and followed by a 

 thorough washing in 70 per cent, alcohol. The material fixed in this 

 manner has proved exceedingly valuable, anil many of my best draw- 

 ings were made from it. The form and relation of the tliffercnt parts 

 are admirably preserved, as well as those of the cells composing them, 

 while in many cases the finer structures, such as cilia, maintain their 

 integrity. After sectioning and staining the individuals collected in 

 the summer of 1903, which wiere fixed by this method, a peculiar 



