No. 2.] THE EMBRYOLOGY OF LIMULUS. 203 



to its late appearance he is not certain as to its nephridial 

 nature. 



Nephridia (so-called Coxal Glands). — As has been described 

 above, a portion of the ccelom of the iifth somite remains upon 

 the ventral side of that somite, while the other coelomic cavities 

 are migrating toward the dorsal surface (Fig. 52). For a con- 

 siderable time this cavity shows no changes worthy of remark, 

 and first in about Stage G, (Fig. 32) is any modification notice- 

 able. The cavity now begins to elongate and to become bent 

 upon itself Hke the letter U, the rounded portion being directed 

 anteriorly. The U now grows in length and, the posterior 

 end being fixed, soon extends into somite IV. With this 

 change in shape a differentiation in the epithelial cells be- 

 comes noticeable, most of them becoming cubical or columnar, 

 while those upon the inner side of the inner end of the U 

 retain their pavement character. 



These changes, just before Stage H, produce the following 

 results (Figs. 56-59): The coelom (of somite V) is now di- 

 vided into two portions, (i) a true coelomic portion (the "end 

 sac" of authors, Fig. 58 nst) bounded by pavement epithelium 

 on its inner side, and passing directly into the nephrostomial 

 portion with columnar cells ; and (2) the nephridial duct which 

 passes forward (Fig. 57), as the proximal limb of the organ, to 

 the loop (Fig. 56) and then backward, as the other limb (Figs. 

 57, 58, 59 nd), to the posterior limits of somite V. As yet 

 it has no connection with the exterior, but in the last section 

 (Fig. 59) can be seen an inpushing of ectoderm {no), which is 

 to form its external aperture. In the various sections in the 

 neighborhood of the coxal gland at this stage may be seen 

 numerous lacunae {lac) in the mesoderm, which is rapidly as- 

 suming the trabecular condition characteristic of the later 

 stages. I have never been able to certainly trace any con- 

 nection between these lacunas and the coelomic cavity and am 

 strongly of the opinion that none exists. 



In Stage H (Fig. ^\ a-i ; Fig. 55, reconstruction) the same 

 conditions are seen, except that the duct now opens to the 

 exterior. We have here the end sac {c 5) which passes directly 

 into the nephrostome without any sharp line of demarcation 



