360 EDWIN S. GOODKICII. 



histologically differentiated, the connecting strands can uo 

 longer Le clearly seen. In late embryos the radial muscles 

 appear to be quite as distinctly separated from each other as 

 in the adult. But in these later stages one can find in 

 sections the network of nerve-fibres which in the adult fin 

 runs along the base of the muscles, and passes from one to the 

 other in a complicated intermuscular plexus. 



It seems, therefore, highly probable that the connecting- 

 strands of embryonic tissue found by Mollier, Brans (4), and 

 myself are really the rudiments of the nerve-plexus. I am, 

 unfortunately, vinable actually to prove this; but there is no 

 doubt that the strands occur before the nerve-plexus can bo 

 found, and at about the stage when we should expect it to 

 develop. At all events, there is no evidence that any muscle- 

 forming cells pass from one muscle-bud to another. 



The Position of the Fins. 



In estimating the exact position of the fins at various 

 stages in development only approximate results can be 

 obtained. It is not possible to compare different stages in 

 the growth of one individual, and there is considerable 

 variation amongst several. Moreover, it is probable that, in 

 the course of growth, segments may become incorporated 

 into the occipital region of the head, where myotomes and 

 their nerves may be reduced or obliterated. We can, there- 

 fore, never make quite certain that a given segment, say 

 the twentieth, in one adult dogfish corresponds to the 

 twentieth segment in another adult, or to the twentieth in 

 an embryo. 



The position of the fins in an adult Scyllium canicula is 

 shown in text fig. 1, and in an embryo, about 19 mm. long, 

 in text fig. 3. In the first the myotomes are not represented; 

 in the second the nerves are omitted, the ganglia only being 

 indicated. 



In enumerating the nerves of the adult the spinal nerve 

 issuing immediately Ijchind the skull was counted as the first. 



