CHAPTER I. DOUBLE MONSTROSITY. 



(For Contents see p. vii.) 



A. LITERATURE. 



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



Salmonidac — d'Audeville 4', Barbieri 6-7; Bugnion 34-', Coolidge 42; Coste 4-3; Dareste 52; 

 Garman and Denton 74; Gemniill 76-77; Girdwoyn 81; Jacobi 108 ; Klaussner 123; Knoch 

 127; Kopsch 132-133; Lovme 146; Moser 165; Oellacher 175-176; Panum 180; Quatrefages 

 197-198; Rauber 200-202; Schmitt 216-7; Schwalbe 222, ii. p. 297 et scq.; Secques 224; 

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



Other Fishes : — 



Perca — v. Baer 5; Leuciscus — Bataillon 11; Annrrichas — Buckland 33a; Girardinus — 

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

 263 ; Blcnnius — 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; Pctromyzo7i—Ba,ta.i\lon 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 

 Salmonidac: 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 mesodidymus. 



On the other hand, v. Baer (5) examined over 3000 eggs of Cyprinus blicca 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 (141-143) 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 



