312 THE CELL 



My brother and I (VII. 20) attempted to cross different species 

 of Echinoderms, and found that when the ova of Echinus micro- 

 tuberculatus were mixed with the spermatozoa of Strong i/locentrotus 

 lividus, fertilisation took place in every case after a few minutes, 

 the egg-membrane raising itself up from the yolk. After an hour 

 and a half all the ova were regularly divided into two. On the 

 following day glistening germ vesicles had appeared ; on the third, 

 gastrul* ; and on the fourth, the calcareous skeleton had deve- 

 loped. Cross- fertilisation in the opposite direction yielded varying 

 results. When the spermatozoa of Echinus micro-tuberculatus were 

 mixed in a watch-glass with the ova of Strongylocentrotus, the 

 greater number of the ova remained unchanged, the egg-mem- 

 brane raising itself from the yolk in only a few cases. After two 

 hours only a few isolated ova were divided into two. Amongst 

 these the egg-membrane lay fairly close to the yolk in some, and 

 in others was raised a little. The next day a few glistening germ 

 vesicles were apparent in the watch-glass, but the majority of the 

 ova were quite unchanged. 



Pfliiger (VII. 50) observed a similar relationship between Rana 

 fusca and Rana esculenta. Ova of the former species, when sus- 

 pended in a watery extract of the testis of Rana esculenta, always 

 remained unfertilised. When, however, the ova of 'Rana esculenta 

 were mixed with the spermatozoa from the testis of Rana fusca, 

 the greater number of the former developed in a regular manner, 

 only a few dividing abnormally; however, after the blastula- 

 stage was reached, they all, without exception, died. 



In many respects the results of hybridisation, seen later in 

 the development of the product of crossing, resemble those of 

 self-fertilisation. For instance, when fertilisation does take place, 

 the embryos in many cases die prematurely, or are of a weakly 

 constitution. 



The embryos, which develop when certain Echinoderms are 

 crossed, do not live beyond the gastrula-stage. In the same way, 

 Pfliiger saw the ova of Rana fusca, which had been fertilised with 

 the spermatozoa of Rana esculenta, die as germ vesicles. The re- 

 productive organs of animal hybrids generally atrophy before the 

 age of sexual maturity is reached, and hence the animals are sterile. 



A still larger number of examples is to be found in the vege- 

 table world. It is true, that seeds may develop, as a result of 

 hybridisation, but they are defective in their development, and 

 sometimes even incapable of germination. If, however, germina- 



