52 



THE STUDY OF CHICK EMBRYOS 



to the vol untary muscle of the bod y. The voluntary or skeletal muscles are thus 

 at first all segmented but later many of the segments fuse. In the trunk muscles 

 of the adult fish the primitive segmented condition is retained. 



The Intermediate Cell Masses or Nephrotomes. — The bridge of cells con- 

 necting the primitive segments with the lateral mesodermal layers constitutes 

 the nephrotom e (Figs. 49 and 53). In the chick the nephrotomes of th e fifth to 

 sixteent h segmen ts give rise dorsad to pairs of small cellular sprouts, the rudi- 

 mentary kidney tubules of the ^ronephroi, segmentally arranged in the fur- 

 row lateral to the primitive segments. By the union of these cell masses dis- 

 tally solid cords are formed which run lengthwise in the furrow. These cords 



Mesonephric dtict 

 Neural tube 



Mesodermal segment 



Somatic mesoderm 



Splanchnopleure- 

 Descending aorta- 



Notochord Entoderm Ccelom 



Fig. S3. — Semi-diagrammatic reconstruction of five mesodermal segments of a forty-eight-hour chick 

 embryo. The ectoderm is removed from the dorsal surface of the embrj'o. 



hollow out, grow caudad, and become the primary excretory {mesonephric) ducts 

 (Fig. 53). More^a udally the intermediate cell mass esjorm the emhrynnir kiH- 

 ji ey or m esonephros, the tubules of which open into the jBriraaxx-exc retorv duc t. 

 Further details concerning these provisional kidneys are given on pages 195-199. 

 Since the genital glands develop in connection with the mesonephros, and the 

 kidney of the adult (metanephros) is partly developed as an outgrowth of the 

 primary excretory ducyth e intermediate cell mass m ay be regarded as the aiilage 

 of thejvr ogenital glands and their ducts . T hese s tructures are thus of mesodermal 

 origin. 



Somatopleure and Splanchnopleure.— In the embryo of seven primitive seg- 

 ments the mesoderm was seen to split laterally into two layers, the somatic 



