A Cytological Study of the Semi- parasitic Copepod, Hersilia apodiformis etc. 427 



the chromosomes of the various species belonging to the genus Notonecta. 

 Federley shows that, whereas there may be a general riile which holds 

 that, m a series of related foriiis, the niost primitive (oldest phylogenetic- 

 ally) have the largest number of chromosomes and the most speciahzed 

 (yomigest phylogcnetically) the smallest number, yet such a rule would 

 not be without exception. The genus Pygaera is such an exception, the 

 series there being just the reverse of such a series as Braun has con- 

 structed for the genus Cydops. Miss Browne also points out that, while 

 in a majority of cases the chromosome number in closely related species 

 er varieties does not show a wide fluctuation, yet there are many 

 exceptions to this rule, such as the various forms of Cydops viridis 

 (Chambers '12 b) and Thyania custator (Wilson '07). Such wide differ- 

 ences in chromosome number can hardly be accounted for either by a 

 fusion of the chromosomes to form complex chromosomes, such as 

 the "butterfly-form" tetrad of Nezara (Wilson '11), or Notonecta (Browne 

 '13), or by the segmentation of a multiple chromosome, such as the long 

 pair in Tachea hortensis (Baltzer '13). 



A comparison of the chromosomes in the biserial arrangement of 

 the European Cydops viridis (Braun '09) with those of Cydops viridis 

 var. parcus (Chambers '12a, '12b) shows that it is quite impossible to 

 explain the reduction of six in the European form to three in the American 

 variety (or species) by a process of fusion. Certainly there is less chromatic 

 substance in the chromosomes of the latter. Our ideas of chromosomes 

 are mostly bound up with a material basis for inheritance. As Federley 

 ('13) has Said, it is their stainability which has brought them to our atten- 

 tion; yet their behavior, not only in mitosis and gametogenesis, but also 

 in hybridization and other experimental studies, proves that they are 

 individuals and that the theory which designates them the bearers of 

 heredity is no fanciful idea. The ideas of Boveri are generally accepted 

 — that a complete set of chromosomes must be present in order that a 

 complete individual may develop. Yet we are almost totally in darkness 

 when we consider such a specific example as the foUowing: "Why is it 

 that it requires eleven chromosomes (haploid number) i) for the produc- 

 tion of an individual of Cydops strenuus, and only two to bear all the 

 determiners necessary to produce an individual of Cydops viridis var. 

 hrevispinosus (ChAMBERS '12 a)? Both are members of the sanie genus 

 and one possesses all the organs borne by the other. There seem to be 

 but two methods by which we can attempt an explanation: (1) either 



1) The haploid mimber is given heie, because it is generally accepted that each 

 gamete brings in a complete set ol: determiners. 



