No. 3. — A Method for Orienting small Objects for the Microtome. 

 By W. McM. WooDWORTH.i 



In studying the embryology of Polychcerus caudata, an acoelous 

 Turbellarian, I experienced great difficulty in obtaining sections in 

 definite planes, owing to the extreme smallness and nearly spherical 

 form of the early stages. Tlie difficulty lay in controlling the plane of 

 section in relation to the axes of the object. The embryos measure only 

 about 0.224 mm. in diameter, and all oi'ientation must be done under low 

 powers of the microscope. I tried orienting in paraffin, which was kept 

 fluid by means of hot water circulating through a combination Strieker's 

 gas and warm stage into the central well of which iced water could be 

 quickly introduced. I hoped thus by the sudden cooling of the paraffin 

 to fix the object in the position into which it had been brought by means 

 of needles. This method proved useless, for owing to their lightness and 

 spherical shape the objects could not be kept long enough in one position, 

 being moved about by the convection currents in the hot paraffin. The 

 method suggested by Born (Zeitschr. f wiss. Mikr., Bd. V. p. 436, 1888) 

 was also tried, but was not applicable to objects so small and round, it 

 being impossible, in early stages, to determine the position of the poles, of 

 the egg owing to the fact that the grooves of the cleavage planes become 

 filled with paraffin and are thus obscured. The method is useful, how- 

 ever, when the embryos have developed a characteristic shape or sym- 

 metry that is visible through the coating of paraffin. I was still at sea 

 with my young stages when Dr. William Patten told me of a method 

 employed by him, in experimenting with which I developed the method 

 which is the subject of this communication. 



The method depends upon the use of paper having a surface of raised 

 parallel lines, or in other words, ha,ving a grained or rep-surface. Writ- 

 ing paper of this kind is made in various sorts and can be had in the 

 market. The best kind is that which is known as "linen cloth," and is 

 made in imitation of somfe coarse fabric. It bears series of parallel raised 

 lines intersecting one another at right angles in imitation of the woof 



1 Contributions from the Zoulogical Laboratory of the Museum of Comparative 

 Zoology, under the direction of E. L Mark, No. XXXVIII. 



VOL. XXV. — NO 3. 



