EMBRYO OF SEVENTEEN SEGMENTS 



S3 



Mesodermal Segments. We have seen that these are developed by 

 the appearance of transverse furrows in the mesoderm (Fig. 53). Later, 

 a longitudinal furrow partially separates the paired segments from the 

 lateral unsegmented mesoderm. The segments are block-like with 

 rounded corners when viewed dorsally, triangular in transverse sections 

 (Figs. 49 and 53). They are formed cranio-caudally, the most cephalic 

 being the first to appear. The first four lie in the head region. The 

 segments contain no definite cavity, but a potential cavity representing 

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

 ments form a thick mesothelial layer about them (Fig. 49). The ventral 

 wall and a portion of the median wall of each primitive segment become 

 transformed into mesenchyma which surrounds the neural tube and 

 notochord (Fig. 290). The remaining portions of the segments persist as 

 the dermo-muscular plates. The cells of the mesial wall of the plate, the 

 myotome, elongate and give rise to the skeletal muscle of the body. These 

 muscles are thus at first segmented, but later many of the segments fuse. 

 In the trunk muscles of the adult fish the primitive segmental condition 

 is retained. 



Mesonephric duel 

 Neural tube 



Mesodermal segment 



Somatic mesoderm 



Splanchnopleure 

 Descending aorta 



Notochord Entoderm Ctelom 



FIG. 53. Serai-diagrammatic reconstruction of five mesodermal segments of a forty-eight-hour 

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



The Intermediate Cell Masses or Nephrotomes. The bridge of cells 

 connecting the primitive segments with the lateral mesodermal layers 

 constitutes the nephrotome (Figs. 49 and 53). In the chick, the nephro- 

 tomes of the fifth to sixteenth segments give rise dorsad to pairs of small 

 cellular sprouts, the rudimentary kidney tubules of the pronephroi, 

 segmentally arranged in the furrow lateral to the primitive segments. By 

 the union of these cell masses distally, solid cords are formed which run 



