212 THE FOURTH DAY. [CHAP. 



The limb-muscles, both extrinsic and intrinsic, are in certain 

 fishes (Elasmobranchii), derived from the muscle-plates, and a 

 similar origin has been observed in Lacertilia and Amphibia. 



In the Chick and other higher Vertebrata on the other hand 

 the entrance of the muscle-plates into the limbs has not been 

 made out (KoUiker). It seems therefore probable that by an 

 embryological modification, of which instances are so frequent, 

 the cells which give rise to the muscles of the Umbs in the higher 

 Vertebrata can no longer be traced into a direct connection with 

 the musclo-plates. 



At first, as is clear from their mode of origin, the 

 muscle-plates correspond in number with the protover- 

 tebrsB, and this condition is permanent in the lower 

 vertebrates, such as fishes, where we find that the 

 lateral muscle is divided by septa into a series of 

 segments corresponding in number with the vertebrae. 



WolflSian body or mesonephros. Of all the events 

 of the fourth day, none perhaps are more important than 

 those by which the rudiments of the complex urinary 

 and generative systems are added to the simple Wolffian 

 duct and body, which up to that time are the sole repre- 

 sentatives of both systems. 



We saw that the duct arose on the second day (pp. 

 94, 106) as a solid ridge which subsequently became a 

 tube, lying immediately underneath the epiblast above 

 the intermediate cell-mass, close against the upper and 

 outer angles of the somites, and reaching firom about 

 opposite to the seventh somite away to the hinder end 

 of the embryo. 



At first the duct occupies a position immediately 

 underneath the superficial epiblast, but very soon after 

 its formation the growth of the somites and the changes 

 which take place in the intermediate cell-mass, together 



