William J. Moenkhaus 39 



velopment proceeded. This slowing-up process is to be interpreted as an 

 increased weakening in the developmental energy. Either the miequal 

 givins: out of, or the unequal draft upon, this energy in different por- 

 tions of the developing eml^ryo, may result in the various abnormalities 

 above described. 



The bearing of these abnormalities upon the question of embryo forma- 

 tion is not to be discussed here. Other crosses have yielded material 

 much more instructive and the subject will be taken up in connection 

 with a description of those crosses. 



VI. The Individuality of the Maternal and Paternal 



Chromosomes. 



1, Introduction. — As stated in the introduction of this work, one of 

 the points of especial interest in the hybrids between Fundulus hetero- 

 clitus and Menidia notata is the fact that the chromosomes of the one 

 may be distinguished, morphologically, from those of the other. I was 

 introduced into the importance of this through the study of a section 

 of a hybrid egg which was in the anaphase of the first cleavage. In this 

 spindle two kinds of chromosomes appeared, easily distinguishable. Sub- 

 sequent comparison with the chromosomes of the parent species showed 

 that one of the kinds of chromosomes belonged to one and the other to 

 the other parent, and that the introduction into a strange egg did not 

 modify their characteristic form. With these conditions obtaining it has 

 been possible for me to follow the history of the maternal and paternal 

 chromosomes in these hybrids to a late stage of cleavage. The phase 

 of this subject which has engaged me especially is that of the individu- 

 ality of the two parental chromosomes during development. 



2. Material and Methods. — Appropriate stages were preserved from 

 the moment of impregnation to a late cleavage stage. Corresponding 

 stages of both hybrids and of normal eggs were taken. The killing fluids 

 used were Flemming's, Zenker's, Perenyi's, and picro-acetic. The last 

 two have been of most service to me. The eggs were directly placed into 

 the fluids without first removing the membranes. The most convenient 

 method for manipulating the eggs in the paraffin and one which I adopted 

 altogether is as follows: the membrane was removed and the yolk with 

 the protoplasmic cap or embryo surmounting it was imbedded with the 

 cap directly upward. By properly mounting the paraffin block I could 

 lay the protoplasmic cap into horizontal sections until the yolk was 

 reached. This very much simplified an, at best, very laborious task. 

 The sections were in practically all cases made 7^2 or 10 micra thick. It 



