CHAPTER I. DOUBLE MONSTROSITY. 



(For Contents see p. mi.) 



A. LITERATURE. 



REFERENCES will be found under the following index figures (pp. xi-xvii) : 



Salmonidae. d'Audeville 4> Barbieri 6-7; Bugnion 34', Coolidge 4%', Coste 43', Dareste S3; 

 Garman and Denton 74; Gemmill 76-77; Girdwoyn 81; Jaeobi 108; Klaussner 123; Knoch 

 137; Kopsch 132-133 ; Lovmel46; Moser 165; Oellacher 176-176; ~Pa,nnml80; Quatrefages 

 197-198; Rauber 200-203; Schmitt 216-7; Schwalbe 222, ii. p. 297 et seq.; Secques 224; 

 Sutton 245; Taruffi 247; Windle 271-2; Yarrell 275, ii. p. 107. 



Other Fishes: 



Perca v. Baer 5; Leuciscus Bataillon 11; Anarrichas Buckland 33a; Girardiniis 

 Emeljanov 65 ; Esox Klaussner 123, Lereboullet 141-143, Rauber 200, Rauber 202, Valentin 

 263 ; Blennius Rathke 199 ; Scomber Sutton 245 ; Selachoidei Aldrovandi 2, v. Baer 5, Gadeau de 

 Kerville 73, Heusner 94, Klaussner 123, p. 12, Levison 144, Lowne 146, Quatrefages 198, Risso 205, 

 St. Hilaire 213, Sutton 245; Torpedo Dohrn 57; Petromyzon Bataillon 12. See also pp. 30-32. 



B. OCCURRENCE, RECORDS, AND GENERAL OUTLINE. 



From the scattered data available, it would seem that the frequency with which double 

 monstrosity makes its appearance in the development of fishes varies in different species and 

 in broods from different parents within the same species. The following figures apply to the 

 Salmonidae: 1 in 50, and 1 in 280 (Rauber 200 and 202); 1 in 600, none in 600, and 68 in 

 900 (Schmitt 216); 1 in 200, and 1 in 350 (author's observation); over 100 in 400,000 (Coste 

 43), Oellacher (176) notes a remarkable brood in which the proportion probably reached as 

 high as fifty per cent., or at any rate twenty to thirty times more than the average. Here, how- 

 ever, the duplicity was of the peculiar and imperfect type described by this author as mesodidymiis. 



On the other hand, v. Baer (5) examined over 3000 eggs of Cyprinus Uicca without result. 

 In Perca fluviatilis he obtained two double embryos out of a set of forty eggs, although a very much 

 larger number (over 1000) in other sets provided none. Lereboullet (141-14$) examined in all 

 203,962 eggs of the pike (Esox lucius), obtaining 222 double monsters, an average of 1 in 920. 

 In the same species Rauber (202) found a single example in a set of 325 eggs, and Valentin (263) 

 six out of 917 hatched embryos. In Petromyzon, Bataillon (12) records an extraordinary case 

 where forty out of a hundred eggs developed twin gastrulae. We owe to Lereboullet (143) the 

 illuminating statement that within the same species (Esox lucius), the prevalent types of monstrosity 

 as well as the frequency with which these occur vary somewhat in different groups of eggs. Much 

 work, however, remains to be done on frequency and type in relation to parentage, alike on the 

 male and on the female side, though some interesting figures bearing on the latter are given by 

 Rauber (202 6, 129-184). 



It is worthy of note that the frequency with which double monstrosity appears in the eggs of fishes, 



A 



