On the Probable Error of Micrometric Measurements. 93 



The cercaria now loses its tail and becomes encysted,* either 

 on or within the body of some mollusk or aquatic animal. 

 This is known as the pupa condition. In this state it passes 

 into the stomach of some higher animal, where it loses its 

 covering and acquires all those organs which entitle it to be 

 called a sexually mature distome. 



Thus we see the embryos, as they escape from the eggs, are 

 not at once transformed into individuals like their parents, 

 but they in turn produce other larva-like forms, capable in 

 their turn of producing still other larvae, which alone resemble 

 the parents. This form of development is called "Alternate 

 Generation." As we have seen, the embryos are sometimes 

 developed before the eggs are cast, and, indeed, may escape 

 from the eggs while they are still within the uterus. 



Physiological Laboratory, 



University of Michigan. 



ON THE PROBABLE ERROR OF MICROMETRIC 

 MEASUREMENTS. 



BY EDW. W. MORLEY, M. I)., PH. D. 



{^Received Nov. 2pt/i, i8j8.) 



To some of the readers of the Quarterly Microscopicai, 

 Journal it may be interesting, or possibly useful, to have an 

 estimate of the degree of accuracy attainable in those micro- 

 metric measurements demanding the highest precision. The 

 statements in this article apply directly only to measure- 

 ments of objects fulfilling certain conditions; they would not 

 apply rigorously to the measurement of anything but fine, 

 straight lines ruled on a transparent medium, and viewed b}^ 

 transmitted light. 



Professor William A. Rogers and the writer have made nu- 

 merous measurements of the intervals between lines ruled on 

 glass plates; the object being to see how near the profess- 

 edly equal spaces were to actual equality. Care was taken to 

 make our results absolutely independent of each other's work, 

 by keeping ourselves in absolute ignorance of each other's 

 results. Professor Rogers has examined the question pro- 



*It is supposed these cysts are formed by means of a special, secreting organ, with the help of 

 the epidermis. 



