GENES AND DEVELOPMENT I47 



which is probably proteolytic, and which gives rise to poisonous 

 phenolic and indolic end-products which are normally neutralized by 

 combination with sulphate ions. 



The egg cytoplasm not only contains the fundamental plan of the 

 developing gastrula, but also seems to control the speed of cleavage. 

 Thus in several hybrids between species in which the eggs cleave at 

 different rates, it has been stated that the rate of cleavage is exactly that 

 of the maternal species and that no influence of the sperm can be 

 found. ^ An example- is a cross between Dendraster $ and Strongylo- 

 centrotus q. In the original species the times from fertiUzation to the 

 first and second cleavages are (at 20° C.) Dendraster 57 and 28 minutes, 

 Strongylocentrotus 95 and 47 minutes. Not only did the hybrids cleave 

 at the faster rate, which excludes the possibiHty that the effect might 

 be due to any sort of injury consequent on cross-fertilization, but even 

 enucleated fragments, fertihzed by Strongylocentrotus sperm, cleaved 

 at the faster rate characteristic of their cytoplasm. This behaviour is 

 not always found, since in crosses between large and small varieties of 

 rabbits, the influence of the sperm on the cleavage-rate can be detected 

 as early as the four-cell stage.^ 



In most echinoderm hybrids, the paternal characters begin to be 

 shown at about the gastrula stage. The position in which the mesoderm 

 cells are given off from the endoderm, and the type of skeleton formed, 

 are usually intermediate between the two parental types, though in 

 some cases (e.g. Echinid x Antedon) even these comparatively late 

 characters are purely maternal, von Ubisch^ has recently re-investigated 

 several cases in connection with his studies on "germ-layer chimaeras," 

 which are obtained by adding the presumptive mesoderm cells of one 

 species to a blastula of another. The foreign mesoderm can be added in 

 various proportions, and if the host mesoderm is removed, forms can be 

 produced in which all the mesoderm belongs to a species other than 

 that of the ectoderm and endoderm; these correspond among animals 

 to pericUnal chimaeras among plants. On comparing the skeletons 

 formed in hybrids between species A and B with those formed in 

 chimaeras containing mixtures of A and B mesoderm, the shape was 

 found to be much the same in both cases, and intermediate between 

 the two parental types. Thus for this late character the chromosomes 

 alone in the hybrids had almost the same effect as the chromosomes and 

 cytoplasm of the added mesoderm in the chimaeras. 



* Rev. Morgan 1934, Schleip 1929. ^ Moore 1933. 



' Castle and Gregory 1929. * v. Ubisch 1937. 



