I04 MCMURRICH. [Vol. XL 



scribed as occurring in Asellus, but there seems to be less 

 definiteness in the number of the cells composing the teloblastic 

 row when it is first recognizable than in that form. 



The development of Porccllio and Arviadillidium has now 

 been carried to a stage in which the germ-layers may be said 

 to be differentiated, though as already indicated it is as yet im- 

 possible to distinguish the liver-endoderm from the mesoderm. 

 This differentiation, as is essentially the case with Asellus, only 

 supervenes at a much later stage, and a, halt may be conven- 

 iently called here. In order, however, to carry the account up 

 to the stage at which it was left in Asellus, an additional figure 

 (Fig. 59) is given, in which the embryo is distinctly recogniza- 

 ble. The figure was drawn from an ovum viewed by direct 

 illumination as an opaque object, and shows the optic lobes dis- 

 tinctly indicated. Posteriorly are a number of scattered cells, 

 in front of which is the row of ectodermal teloblasts (7^), which 

 are smaller in comparison with the cells of the teloblastic rows 

 (7>) to which they have given rise than is the case in Asellus. 

 There are about twenty-three teloblasts in the preparation 

 figured, and it is noticeable that there is no indication of the 

 existence of eleven primary teloblasts such as occurred in 

 Asellus and probably also in Jaera, the number of cells in the 

 most anterior of the four teloblastic rows which are present 

 being but slightly less than that of the posterior row. This is 

 in harmony with the indefiniteness of the number of cells com- 

 posing the row in earlier stages, and it may be supposed that in 

 Armadillidium and Porcellio, the teloblasts do not enter upon 

 their characteristic method of division until nearly the entire 

 row has differentiated. No difference can be distinguished be- 

 tween the central and the remaining teloblasts, nor does the 

 median teloblastic row of cells differ from the remaining rows; 

 and furthermore it seems that in the species under considera- 

 tion the scattering of the mes-endoderm takes place relatively 

 earlier in comparison with the formation of the teloblastic rows 

 than in either Asellus or Jaera, since in the stage figured there 

 is no indication of any special mass of mes-endoderm remaining 

 in what was originally the mes-endoderm region, but the scat- 

 tering has been complete. In the naupliar region of the 



