62 



THE STUDY OF CHICK EMBRYOS 



dorsally, triangular in transverse section (Figs. 47 and 51). They are formed 

 cranio-caudally, the most cephalad being the first to appear. The first three lie 

 in the head region. The segments contain no cavity but a potential cavity repre- 

 senting a portion of the coelom is filled with cells, and the other cells of the seg- 

 ments form a thick mesothelial layer about them. The ventral wall and a por- 

 tion of the median wall of each primitive segment become transformed into 

 mesenchyma which surrounds the neural tube and notochord (Fig. 282). The 

 remaining portion of the segments persist as the dermo-muscular plates or myo- 

 tomes. The cells of the myotomes elongate and give rise to the voluntary muscle 

 of the body. The voluntary or skeletal muscles are thus at first all segmented 



Mesonephric duct 



Neural tube 



Mes. segment 



Som. mesoderm 

 Splanchnopleure 

 Descending aorta 



Notochord Entoderm 



FIG. 51. Semi-diagrammatic reconstruction of five mesodermal segments of a forty-eight-hour chick 

 embryo. The ectoderm is removed from the dorsal surface of the embryo. 



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 nephrotome (Figs. 47 and 51). The nephro tomes give rise dorsad to pairs 

 of small cell masses segmentally arranged in the furrow lateral to the primitive 

 segments. By the union of these cell masses solid cords are formed which run 

 lengthwise in the furrow. These cords hollow out, grow caudad, and become 

 the primary excretory (mesonephric) ducts (Fig. 51). The rest of the intermediate 

 cell mass becomes the embryonic kidney or mesonephros, the tubules of which open 

 into the primary excretory duct. As the genital glands develop in connection 



