EXPLANATION OF PLATES XI AND XII. 



Fig. 11. Portion of a transverse sectiou through the region of one of the 

 posterior legs of P. capensis just before birth. The section passes 

 through the opening of the nephridium and the rudimentary crural 

 gland. The tubular part of the nephridium is cut in two places, and 

 its terminal portion with the closely-packed nuclei is shown opening 

 into the internal vesicle. The cavity of the leg is traversed by a con- 

 siderable amount of muscular tissue external to the nephridium, and 

 contains some of the reticular tissue in its dorsal part. The septum 

 separating the lateral sinus from the leg cavity is absent in this region. 



Fig. 12. Section through the ovaries of an embryo of P. capensis just 

 before birth. On the right side tliere is an attachment to the peri- 

 cardium. The dark elongated nuclei are follicular nuclei. 



Fig. 13. Dorsal part of a transverse section of a male of P. capensis just 

 before birth. In most embryos of tin's age the wall of the heart is 

 separate (always connected by filaments) from the dorsal body wall and 

 the pericardial floor. The apparent fusion in this section was pro- 

 bably due to contraction of the body. The two patches of dorsal 

 longitudinal muscles are shown. 



Figs. 14 — 17 are a series of diagrams to show the relations of the eoeloni 

 and body cavity at successive stages. The lining of the coelom is shaded dark, 

 the light shading indicates the general mesoderm. 



Fig. 14. Earliest stage ; coelom as a series of separate spaces. The ecto- 

 derm and endoderm still in contact : no trace of the vascular space or 

 hEemoccele. 



Fig. 14«. The endoderm has already separated from the dorsal and ventral 

 ectoderm. On the right-hand side the somite has not yet divided into 

 a dorsal and ventral portion. The first rudiment of the lateral sinus 

 is present in the thickened mesoderm. On the left side the ccelom has 

 divided into a dorsal part, which in the anterior part of the body 

 vanishes, but in the posterior part becomes the generative organ; 

 and into a ventral part which becomes a nephridium. The lateral sinus 

 has increased in size, and a space has appeared in the mesoderm of 

 the leg. 



Fig. 15. The dorsal division of the coelom has passed dorsalwards, and 

 considerably encroached upon the dorsal of the two blood-spaces formed 

 by the separation of the endoderm and ectoderm. This median dorsal 

 blood-space becomes the heart. Two spaces have appeared in the 

 ventral wall of the dorsal division of the somites ; the dorsal of these 

 becomes the pericardium, the ventral the dorsal part of the central 

 compartment of the body cavity. The ventral blood-spaces have in- 

 creased in size, but are otherwise unchanged. 



Fig. 16. The dorsal divisions of the somites have relatively diminished in 

 size, and been overlapped dorsally by the greatly increased pericardium. 



