36 The Chromatin in the Development of Hybrids 



more rapidl}'^ than those of Fundulus. This increased rate is also main- 

 tained for the Menidia hybrid eggs. 



This law of the rate of cleavage in hybrids I have considered else- 

 where but the following facts are of interest here. When the two species 

 crossed have eggs that cleave at a different rate the cleavage is still that 

 of the egg species. The eggs of Fundulus heteroclitus can very easily 

 be impregnated by Tautoglahrus adsperus. The eggs of the former 

 cleave ordinarily in about two hours after the addition of the sperm. 

 Those of the latter, under similar conditions, cleave in about fifty min- 

 utes. In the hybrid, however, the rapid sperm is unable to alter the rate 

 of the cleavage and vice versa. This law is further strikingly illus- 

 trated in the cross between Batrachus tau and Tautoglahrus. The eggs 

 of the former species can be impregnated by the sperm of the latter. 

 'The cleavage furrows, however, do not appear until 8 hours after im- 

 pregnation, approximately that of the egg species. 



Stassano, 83, maintained that he was able to hasten or retard the 

 cleavage of Echinoderm eggs by sperm of another species. Driesch, 98, 

 however, by extended experiments in the same group of animals, has, 

 shown just the reverse and it is probable that Stassano erred in his ex- 

 periments. 



2. Development of Dispermic Eggs. — The dispermic eggs fall at once 

 into four cells. The cleavage takes place synchronously with the cleav- 

 age of the normal eggs, so that when the normal eggs are in the two- 

 cell stage, about an equal number of eggs will be found in the four-cell 

 stage. This correspondence in the rhythm of cleavage is not strictly 

 maintained after the first cleavage, in that the rate is slightly slower in 

 the normals. The form of the cell cannot be distinguished from those 

 in the four-cell stage of the normals. The four-cell stage, or the first 

 cleavage is followed by the eight-cell stage, this by the sixteen-cell stage, 

 etc., in a normal manner. 



Such dispermic eggs continue their development to a late stage of 

 cleavage, when they invariably die. I have isolated, in the aggre- 

 gate, many hundred dispermic eggs and followed their development but 

 have never seen an egg that showed any signs of forming the germ ring 

 or the embryonic shield. They form a normal heap of cells and the 

 blastoderm may even spread to a slight extent, but beyond this they do 

 not go. 



That such eggs which fall at once into four cells are dispermic, i. e., 

 eggs whose nucleus conjugates with two male pronuclei, is clearly shown 



