.a 





A STUDY OF THE CHROMOSOMES OF THE GERM CELLS OF METAZOA. 155 



tion of all the problems ; an investigation of a number of forms always shows that some 

 are more favorable than others for answering certain questions, and then there is the 

 chance that a wholly unexpected discovery may be made that may have great signifi- 

 cance. So the plea is made here for the comparative method in Cytology, and Cytology 

 should not be ranked as a line of work separate from others it is all Morphology in the 

 broad sense of the term, and it only happens that in Cytology we use higher magnifica- 

 tion powers of the microscope than in other lines. If one form shows phenomena that 

 seem inexplicable after careful work, then the proper method, the one that would promise 

 a surer reaching of results, is not to reexamine this form again and again, but to compare 

 other forms in the search for the explanation. 



In the present paper the part containing the general conclusions may appear dispro- 

 portionally great to the record of the observations. These observations are to great 

 extent on the number and valence of the chromosomes and chromatin nucleoli from the 

 time of the last generation of the spermatogonia up to the formation of the spermatids. 

 But the determination of these numbers is very difficult ; large numbers of sections have 

 to be examined in order to find the necessary stages, and the number of the chromosomes 

 of each stage have to be counted in a considerable number of cells of each species in order 

 to insure accuracy. The counting has been done in all cases by selecting those cells in 

 which the chromosomes are most loosely grouped, being sure at the same time that all 

 the chromosomes are in the plane of the section, drawing the chromosomes carefully with 

 the camera lucida, then counting their number on the drawings. This demands much 

 patience and time, necessitating also constant reexaminations and study of new material, 

 though the results may be tabulated in a very small space. Of course the difficulties are 

 most pronounced where the chromosomes are numerous and small. 



The material was collected by me at two localities in the vicinity of Philadelphia, 

 Pennsylvania, and in the neighborhood of Wood's Holl, Massachusetts. Great care was 

 taken to insure accurate identification of the species, and my specimens were directly 

 compared by me with the collections in the museums of the Wagner Institute of Science 

 and of the Academy of Natural Sciences of this city ; these collections had been labeled 

 by Dr. P. R. Uhler, of Baltimore, our foremost American authority on this group of 

 Insects ; and I must also acknowledge my indebtedness to Dr. Uhler for kindly identify- 

 ing a number of species which were not represented in the collections just mentioned. 

 To my friend, Mr. C. W. Johnson, curator of the Wagner Institute, my thanks are also 

 due for aid in identification. The differences of the spermatogenetic phenomena of 

 different species shows how important it is to secure accurate identification. 



The testes were removed as rapidly as possible from the living animals and immedi- 

 ately placed in the fixing fluids, Flemming's chronio-aceto-osmic acid mixture (the 



