190 THE DEVELOPMENT OF THE CHICK 



muscles of these parts. The voluntary muscles of the head, on 

 the other hand (excepting the hypoglossus musculature), arise 

 in front of the somites; the mesoblast from which they arise is, 

 however, part of the original paraxial mesoblast, in large part 

 at least. It is important to note that the voluntary muscles 

 are epithelial in origin. The involuntary, or smooth, muscle 

 fibers, on the other hand, are mesenchymal in origin. 



The dermatome remains epithelial in all the somites well 

 into the third day; the cells then begin to separate and form 

 mesenchyme; this process begins at the anterior somites and 

 proceeds backwards. The mesenchyme thus formed is the 

 foundation of the derma. 



The Intermediate Cell-mass or Nephrotome. This is the 

 cord of cells uniting somite and lateral plate; it reaches its typical 

 development only from the fifth to the thirty-third somites, in 

 which it contributes to the development of the excretory system. 

 Behind the cloaca, that is in the region of the tail, there is no lateral 

 plate and no nephrotome. 



Origin of the Excretory System. The history of the excretory 

 system in Amniota is of particular interest, because it shows a 

 succession of three separate organs of excretion or kidneys, the 

 first of which is a mere functionless rudiment, the second is the 

 principal organ of excretion during embryonic life (at least in 

 reptiles and birds), and the third finally becomes substituted 

 for the second, which degenerates and is mostly absorbed; 

 however, parts of the second remain and contribute to the 

 formation of the organs of reproduction. The first, known as 

 the head kidney or pronephros, is probably homologous to the 

 permanent kidney of Amphioxus; the second or mesonephros, 

 is the homologue, in part, of the permanent kidney of Anamnia, 

 and the third or metanephros is the permanent kidney. The 

 secreting parts of all arise from the intermediate cell-mass, though 

 not in the same manner. The development of the metanephros 

 does not begin until the fourth day; it is therefore not considered 

 in this chapter. 



Pronephros and Wolffian Duct. The pronephros extends 

 over only eleven or twelve somites, viz., from the fifth to the 

 fifteenth or sixteenth inclusive; it consists originally of as many 

 parts or tubules as the somites concerned. Each tubule arises 

 as a thickening of the somatic layer of the intermediate cell- 



