No. 3-] 



LIMULUS POLYPHEMUS. 



467 



somites are found numerous blood corpuscles, but free cells are 

 never found at this stage inside the somites. 



The chelarial somite is large and distinct near the median 

 line (PI. XXII, Fig. 9, 5<9'), farther away from the median line 

 it becomes constricted and nearly disappears (PI. XXII, Fig. 

 II, 5(9'), and still farther away it again becomes more dis- 

 tinct (PI. XXII, Figs. 12 and 13, SO'). 



The genital duct at this stage is considerably enlarged. 

 It forms a diverticulum on the ventral wall of the opercular 



CR 



Fig. I, — A diagram showing the relative positions and shapes of the cartilage, the genital duct, 

 and the somite in the operculum. The somite is a large cavity at the base of the oper- 

 culum, closed at the median side and extending on the surface of the yolk beyond the 

 outer ectodermic margin of the appendage. On its ventral margin the somite pushes 

 down into the space in the appendage and at the same time bends towards the median line, 

 making a closed diverticulum. The diverticulum lengthens and becomes relatively 

 smaller, and breaking free from the somite it finally becomes the genital duct {g.d.). On 

 the ventral side of the diverticulum is a band of cells which form the cartilage rod (c.r.). 

 Around the base of the operculum and on the ventral side of the somite is a ring of 

 mesoderm cells. On its median and lateral sides it is continuous with the ectoderm. 



somite, with its blind end directed towards the median line. 

 By comparing the different sections of this series it will be 

 seen that the opercular somite is swollen at its median end 

 (PI. XXII, Fig. 9, SO^)y and reduced to a narrow tube on the 

 median side of the point where the genital duct arises from it 

 (PL XXII, Figs. II and 12, S(7). On the lateral side of this 

 point the somite again enlarges to form a spacious chamber 

 (PI. XXII, Figs. 16-20, SO'). 



The relations of the genital duct, appendage, mesodermic 

 ring, and cartilage to the opercular somite are shown in the 

 accompanying diagrammatic cut (Fig. i). 



