30 The Chromatin in the Development of Hybrids 



portant question of the individuality of the maternal and paternal chro- 

 mosomes during the development of the hybrids. Before taking up this 

 question, a brief description of the impregnation, cleavage and later devel- 

 opment of these crosses will be given. 



Among the many to whom I am under obligations for favors, I wish 

 especially to mention Professor Charles B. Davenport, not only for 

 much help during the progress of the w^ork, but also for first directing my 

 attentions to the possibilities in this line of experimentation. I wish 

 also to especially thank the United States Commissioner of Fish and 

 Fisheries, Geo. M. Bowers, for continual privileges at their Woods Hole 

 Marine Station. 



II. Materia^ and Methods. 



Fundulus heteroclitus and Menidia notata are among our most com- 

 mon coast fishes. Both species can be obtained in any desired number 

 in the bays along our eastern coast. They spawn over a period of about 

 six weeks, beginning the latter part of May. The spawning period 

 must be about the same, since I have always been able to obtain ri]5e 

 individuals of both species at the same time during the period above 

 mentioned. The eggs of Fundulus heteroclitus are the larger, measuring 

 13-14 to the inch. I have taken as many as 599 eggs from a single large 

 female, but the number obtained is usually considerably smaller than 

 this. The eggs of Menidia notata measure on the average 26 to the 

 inch. It is easy to get several hundred eggs from a single female. A 

 rather large, well-filled female yielded, by actual count, 1413 eggs. The 

 eggs of both species flow readily if properly handled. Those of Menidia 

 often flow so easily as to make it difficult to handle a ripe individual 

 without losing a portion of the spawn. Menidia notata has a much 

 greater abundance of milt, so that it can easily be expressed as a thickish, 

 perfectly white fluid. It is less easy to express the milt from Fundulus 

 heteroclitus, so that I have usually found it preferable in my experi- 

 ments to cut out the testes and tease them apart over the eggs. The two 

 species belong to two distinct orders, Fundulus to the Haplomi and 

 Menidia to the Acanthopteri. 



The eggs were, in all cases, fertilized in small watch-glasses. All the 

 eggs desired for any given experiment were first expressed into this 

 watch-glass. Sometimes the eggs of a number of females were placed 

 together when a large lot was desired. The milt was then added and 

 after ten or fifteen minutes the contents were emptied into a finger- 

 bowl of fresh sea-water. By a series of washings the excess of milt and 

 the defective eggs were removed. The water was renewed two or three 



