25O A. RICHARDS. 



was shown by Miss Morris following the lines of the classical 

 work of Moenkhaus. 



In 1904 Moenkhaus's well-known paper appeared announcing 

 the independence of two chromosome groups on the spindles of 

 dividing eggs of Fundulus and Menidia hybrids, and the recog- 

 nition, based on morphological evidence, of the two groups as 

 from the male and female parents respectively. Conklin (1901) 

 had already recognized the male and female halves of Crepldula 

 cleavage nuclei; Ruckert, (1895) and Hacker (1895, 1902) had 

 seen the double nuclei and bilateral distribution of chromatin 

 in Cyclops; and Herla (1893) and Zoja (1895) had traced the inde- 

 pendent maternal and paternal chromosomes of Ascaris eggs to 

 the twelve-cell stage. Moenkhaus, after identifying in the 

 normal eggs the chromosomes of Fundulus as straight rods and 

 those of Menidia as shorter curved rods, easily distinguished the 

 male and female contributions to the cleavage nucleus of the 

 hybrid eggs. 



Since then a great deal of work has been done on the hybridiza- 

 tion of many animals, and some further studies have been made 

 on the crossing of different species of fish. In this group of 

 animals, however, cytological observations have not been much 

 extended. Yet G. and P. Hertwig figure a double spindle 

 showing the portions derived from the two parents, and Miss 

 Morris has corroborated Moenkhaus's principal findings in her 

 crosses of Fundulus and Ctenolabrus. In the latter case, the 

 Fundulus chromosomes, which are small straight rods, as Moenk- 

 haus found them, differ clearly from those of Ctenolabms, which, 

 according to Miss Morris, are small and round. 



At Woods Hole, 1 in the summer of 1914 I repeated these experi- 

 ments of Miss Morris on crossing the eggs of Fundulus with 

 Ctenolabrus sperm. Since the reciprocal cross is not easily made, 

 and since the fact as to whether the cross is possible was not 

 under investigation, my experiments were limited to crossing in 

 the one direction. It had occurred to me that some light might 

 be thrown on the question of the effects of radioactivity on 

 chromatin by treating variously with X-rays the eggs and sperm 



1 1 am greatly indebted to the Director of the 'Marine Biological Laboratory 

 for his kind assistance in providing me with the facilities of the laboratory. 



