450 H. H. NEWMAN 



group sharply marked off from the maternal group. Subse- 

 quently, however, the two types intermingle, though still retain- 

 ing their specific characters. The rate of cleavage was, accord- 

 ing to Moenkhaus, the same as in pure bred eggs of the same 

 species. It is a noteworthy fact that when Fundulus eggs are 

 fertilized with Menidia sperm, the period of gastrulation is safely 

 passed and a well-defined, though short and defective embryo 

 is formed although the two species represent two distinct orders 

 of teleosts. 



Kammerer ('07) published a rather extensive account of hy- 

 bridizing two species of perch, Perca fluviatilis and Acerina cernua. 

 In brief his report is as follows. In the spring of 1905 a fisher- 

 man brought to the laboratory two specimens of an unknown 

 type of perch, which, at the suggestion of Doctor Przibram, were 

 adjudged to be hybrids between Perca fluviatilis and Acerina 

 cernua, because in a number of respects they appeared to be 

 intermediate between these two species. In order to test the 

 correctness of this diagnosis it was decided to intercross the 

 hypothetical parent species in order to determine whether the 

 hybrids resembled the questionable wild individuals. It was 

 found that the two species did cross successfully and that a 

 number of young fish hatched and grew well for about six months, 

 at which time they showed some characters which slightly re- 

 sembled those of the supposed adult wild hybrids. The latter 

 were successfully crossed, since they were both fertile females, 

 with males of both Perca and Acerina, the young resembling 

 closely the paternal species in each case. This is the only case 

 on record, so far as I am aware, .of fertile hybrids being produced 

 by crossing two genera. It is rare, in f^ct, to find undiminished 

 fertility in hybrids between two closely related species of the 

 same genus, and even inter-varietal hybrids frequently show 

 incomplete fertility. Had Kammerer reared to maturity and 

 successfully bred the unquestioned hybrids which he obtained he 

 would have made good his case, but as the matter stands there 

 is considerable cause to doubt the vahdity of his conclusions. 



The chief value of the paper in the present discussion lies in 

 the fact that there is presented a considerable array of data, 



