No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 115 



may be necessary before differentiation can take place, this re- 

 lationship, on account of the greater quantity of yolk in Asel- 

 lus, supervening later in that form than in Jaera. This sug- 

 gestion is tempting, but it must be noted that a further retarda- 

 tion of the differentiation does not take place in Arniadillidiiim 

 or Porcellio, whose ova are again several times the size of those 

 of Asellus, and yet the differentiation is identical in time with 

 that of Asellus. 



Part III. — The Later Development of the Germ-layers. 



I . The Later History of the Mesoderm. 



As has been already pointed out, two distinct regions are to 

 be recognized in the Isopod embryo, an anterior one corre- 

 sponding to the naupliar region and a posterior or meta-nau- 

 pliar region. In these two regions one finds a very different 

 behavior of the mesoderm. In very young embryos only the 

 naupliar region is represented, the blastoporic region lying 

 immediately behind it, the front edge of the blastopore being 

 formed by a row of ectoderm cells, in some cases at least, 

 eleven in number, while the ectoderm cells in front of this are 

 arranged more or less irregularly, though the orthogonal curves 

 described by Reichenbach ('86) in Astaciis are more or less 

 plainly visible. 



In the earliest stages this ectoderm rests directly upon the 

 yolk, there being no trace of mesoderm or ectoderm below it. 

 As described in preceding pages, the cells of the blastopore 

 area, the mes-endoderm and the vitellophags, immigrate and 

 multiply, forming a plug projecting into the yolk, and later 

 they scatter, principally forward, so that at this stage numerous 

 cells begin to be found beneath the naupliar ectoderm. In 

 none of the forms studied is there to be observed any differ- 

 ence in form or appearance between the mesoderm and the 

 endoderm cells at this stage ; in Jaera the vitellophags are 

 early distinguishable from the other mes-endoderm cells, but 

 in other forms they do not become distinguishable until the 

 beginning of the scattering of the mes-endoderm plug when 



