No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA, iij 



no distinction of a secondary mesoderm from a primitive one 

 at this stage, though later, I believe, cells homologous with 

 Reichenbach's secondary mesoderm are to be found. 



The later development of the naupliar mesoderm I have not 

 attempted to follow, and shall turn now to a consideration of 

 what takes place in the meta-naupliar region. 



At the time of the scattering of the mes-endoderm cells the 

 ectodermal teloblasts begin to divide in the manner described 

 and grow backwards over the blastoporic region, the entire 

 meta-naupliar ectoderm on the ventral surface being produced 

 by this teloblastic growth. In embryos which are removed 

 from the yolk, it can readily be seen that the meta-naupliar 

 mesoderm, as well as the ectoderm, is produced by a teloblas- 

 tic growth, though the rhythm of division of the mesodermal 

 teloblasts and their arrangement is very different from that of 

 the ectoblasts. I have succeeded in making out these meso- 

 blasts in Porcellio, Ligia, and CymotJioa with great distinctness, 

 and sections of Jaera and Aselliis show that essentially the same 

 arrangement of them occurs in these forms. 



In Fig. 62 is represented a preparation of Ligia, which shows 

 the arrangement occurring in that species, and I have chosen 

 Ligia for this purpose on account of the discrepancies which 

 my preparations show when compared with the figures given 

 by Nusbaum ('93, Fig. 6). Preparations exactly similar in 

 all essentials have been obtained from CymotJioa and Poixcllio, 

 and also by Bergh from Mysis, a fact which strongly suggests 

 that Nusbaum's observations are somewhat incorrect. Lying 

 beneath the ectoblasts, and slightly behind them, are eight 

 mesoblasts arranged in a very definite manner. One is situ- 

 ated on each side of the median line of the embryo; at a slight 

 distance from these on each side are three others, which are 

 separated from one another by intervals shorter than that 

 which separates the innermost one from the cell which lies 

 near the middle line. Immediately in front of each one of 

 these mesoblasts is to be seen a single cell, the products of 

 their last division, and further forward are to be seen three 

 other transverse rows, each consisting also of eight cells, 

 arranged as in the two posterior rows. In the sixth row, 



