98 THE PHYSIOLOGY OF TWINNING 



TWINNING IN OTHER CHORDATES 



Cyclostomata. — Only one other class of vertebrates 

 remains in which twinning has not already been dealt 

 with: the round-mouth eels. That twinning occurs here 

 as elsewhere among the vertebrates is evidenced by the 

 fact that several papers have been written on twinning 

 in these forms. 



Barfurth (1899) has described a case of a larva of 

 Petromyzon planeri with two tails. Bataillon has written 

 a note on spontaneous blastotomy and conjoined-twin 

 larvae in the lamprey. 



• Elasmohranchii. — Dohrn (1902) has described an 

 interesting double Torpedo embryo which is of especial 

 interest because it is so clearly a product of partial fission. 

 There are two complete notochords but only one median 

 row of mesoblastic somites, belonging equally to the 

 two half-embryos. The outer sides of the two half- 

 embryos are quite complete and exactly equivalent 

 mirror-images of each other. Kaestner (1898) reports 

 the finding of two eggs of the selachian Pristiurus which 

 had two blastoderms on a single yolk. In one of the 

 eggs the two blastoderms were of equal size and about 

 one-fourth of an inch apart; in the other egg the two 

 blastomeres were of very different size and in contact 

 as though one had been split off from the other. 



Amphioxus. — While Amphioxus is not a true verte- 

 brate it is believed to be the most closely allied of the 

 chordates to the vertebrates. It is therefore of interest 

 to record briefly in this place the well-known work of 

 E. B. Wilson on artificial production of twins in Amphi- 

 oxus. By shaking the eggs while in the two-cell stage 

 the blastomeres are either entirely separated so as to 



