ARTICLE IV. 



A STUDY OF THE 0HE0MOSOMES OF THE GERM CELLS OF METAZOA. 



Plates IV— VIII. 

 I'.Y TIIOS. II. MONTGOMERY, .11:.., PH.D., 



ASSISTANT PBOFESSOB OF ZOOLOQY, I'Nl vliltslTY OK PENNSYLVANIA, rim.. Mini. rill A . 

 Read January 18, 1901. 



I. Introduction. 



The present study is practically a continuation of previous work of mine upon sper- 

 matogenesis in the Arthropods. It was undertaken primarily to correct certain errors of 

 interpretation and observation in my work on Pentatoma (Michistus). But many mor- 

 phological problems arose in connection with this reexamination, such as the significance 



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of the changes in the synapsis stage, the significance of the chromatin nucleoli, tl 

 reasons for a. reduction division, the significance of the sequence of the stages of the 

 germinal cycle, and the question as to why different species have different numbers of 

 chromosomes. Thus my investigations given here are essentially on the history of the 

 chromosomes during the germinal cycle. 



It is impossible to answer these problems by an examination of a. single species, and 

 accordingly there are presented here the results of a, comparative study of the spermato- 

 genesis of some forty-two species of Hemiptera heleroplera, belonging to twelve different 

 families. This comparative study has brought to light certain wholly unexpected phe- 

 nomena, and none less anticipated than the discovery of four species with a.n uneven 

 normal number of chromosomes ; this discovery has furnished facts for explaining how 

 the chromosomal numbers may change with the evolution of the species, and how the 

 chromatin nucleoli may have originated. And only such a, comparative study could 

 furnish facts to show that in the synapsis stage bivalent chromosomes are formed by the 

 union of paternal with maternal chromosomes — i. e., that this is the stage of conjugation 

 of the chromosomes. The comparative method in Cytology cannot be overestimated, 

 hough of course careful detailed examinations of single objects should be carried on at 

 the same time. For a. single object is rarely capable of serving as the basis of explana- 



