466 PATTEN AND HAZEN. [Vol. XVI. 



The first traces of the genital ducts were found in embryos 

 having three abdominal somites (PI. XXII, Figs. i-8). In this 

 series the sections begin near the median line and extend 

 towards the lateral ends of the somites. Near the median line 

 are a few loose mesoderm cells. Those near the surface of the 

 yolk are undergoing degeneration. Their nuclei contain large 

 granules that stain deep red in borax carmine, and the nuclear 

 membrane seems in some cases to have ruptured. 



The lumen of each somite increases in size as one moves 

 away from the median line. At the same time the somatic 

 layer increases rapidly in thickness, till just beneath the append- 

 ages it is many cells deep. The splanchnic layer contains only 

 a few isolated nuclei. The lateral ends of the somites have no 

 lumen and consist of mesoderm cells not arranged in distinct 

 layers. 



About midway between the median and lateral ends of the 

 opercular somite the genital duct appears as a long diverticulum, 

 extending from the coelom into the somatic layer (PI. XXII, 

 Fig. 6, g.d.). A similar diverticulum, although not as marked, 

 is shown in the first gill in PI. XXII, Fig. 5. It was not deter- 

 mined whether this was a rudimentary genital duct or the begin- 

 ning of the branchial cartilage. 



In PI. XXII, Figs. 2 and 3, a body resembling an extra 

 somite is present between the operculum and the first gill. Its 

 median end was hollow, while its lateral extremity was united 

 with the mesoderm of the first gill (PI. XXII, Figs. 2-4, SO"). 

 Such a structure was not seen in any other series of sections 

 of this age. 



In the next stage five abdominal somites are present. They 

 are shown in PI. XXII, Figs. 9-20, which represent a series of 

 longitudinal sections beginning near the median line and extend- 

 ing almost to the lateral ends of the somites. 



In the opercular and first gill segment the somites have moved 

 towards the yolk, leaving behind at the base of each appendage 

 a tJiick ring of 7nesoderm, derived from the somatic mesoderm 

 (PI. XXII, Figs. 12-20, SO'-SG^). As the appendage grows 

 in length, a space is formed between its "ectoderm and the ring 

 of mesoderm at its base ; and in these spaces and around the 



