No. I .] EM BR \ 'OL OG Y OF THE ISOPOD CR USTA CEA . I 2 3 



come separated by a considerable distance from the nerve 

 ganglia, a separation which increases markedly in the succeed- 

 ing stages. Beneath the point of origin of each limb is to be 

 seen a portion of the limb mesoderm, and in the transverse 

 section of the maxilliped a core of mesoderm occurs which has 

 evidently wandered into the limb as it grew out from the body. 

 Laterally to the limb mesoderm on each side, is a portion of 

 the lateral mass of the segment, still distinct, and the only 

 marked difference from the section shown in Fig. 65 is the ab- 

 sence of the more median mesoderm masses, which are repre- 

 sented only by a few scattered cells situated on either side of 

 the nervous cord. This is perhaps to be explained by the sep- 

 aration of the limbs from the median line which has already 

 been mentioned, the mesoderm having scattered so as to cover 

 the greater surface now developed. 



As to the ultimate fate of the various portions of the meso- 

 derm, it may be stated definitely that the limb mesoderm and 

 the mesoderm of the lateral masses become converted almost 

 entirely into muscle tissue, though the possibility of a certain 

 amount of connective tissue being formed from them cannot 

 be excluded. The fate of the median masses could not be fol- 

 lowed, however, though I am inclined to think that they form 

 connective tissue rather than muscle fibers. Consequently I 

 have refrained from applying Bergh's ('93) term of myoblasts 

 to the mesodermal teloblasts of the meta-naupliar region, since, 

 though applicable to the three lateral mesoblasts of each side, 

 it is questionable whether it is as properly applied in the case 

 of the two median cells. 



In his preliminary paper, Nusbaum ('91) describes with a 

 figure an embryo of Ligia, in which the abdominal limbs are 

 not yet formed, and calls attention to the existence of a transi- 

 tory exopodite in the thoracic appendages. This observation I 

 can confirm, and may add that the exopodites may also be seen 

 in embryos of other Isopods, though less distinctly than in 

 Ligia. Nusbaum also calls attention to a patch of cells in 

 each segment situated just laterad of each limb rudiment, de- 

 scribing them as ectodermal thickenings and homologizing them 

 with epipodites. These patches are readily discernible in the 



