42 THE PHYSIOLOGY OF TWINNING 



points upon one germ ring. They therefore must have 

 come from two separate blastoderms. The two blasto- 

 derms probably arose through the physiological isolation 

 of the first two blastomeres, as so frequently happens in 

 parthenogenetic Patiria eggs. This mode of origin of 

 fish twins must be extremely rare, for no other such 

 cases are on record. 



Gemmill is of the opinion that the third mode of 

 twinning is found in fishes only in connection with ' ' the 

 peculiar and imperfect doubling characteristic of the 

 hemididymous condition." Two phases of hemididymus 

 are distinguished: (a) mesodidymus, in which there is 

 apparent doubling of the middle region while the anterior 

 and posterior ends remain single; (h) katadidymus, in 

 which the anterior ends remain single and the posterior 

 ends are double. 



As compared with anadidymus, in which the anterior 

 end is double while the posterior end is single, the two 

 forms of hemididymus are extremely uncommon, and are 

 rarely if ever found in advanced embryos. They seem 

 to be due to a mechanical pulling apart of the bilateral 

 primordia during germ-ring overgrowth. Kopsch was 

 able to get katadidymus in trout eggs by injuring the 

 blastoderm at the posterior end of the embryo. Knoch 

 found instances of katadidymus in cases where eggs were 

 rather roughly handled by violent stirring. In general 

 the condition is more like spina bifida than true twinning 

 and may, I believe, be dismissed without further con- 

 sideration. Gemmill's fourth mode of twinning is 

 illustrated for the fishes by a single example of a salmon 

 embryo, reported by Barbieri (1906), in which there is a 

 marked tendency for ventral organs to show greater 



