No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 107 



The next stage which I have seen is one apparently slightly 

 younger than that of which Nusbaum has represented sections 

 in Figs. 18 and 20 of his Polish paper. In surface view there 

 is seen to be a distinct blastoderm which resembles closely the 

 stage of PorcelUo shown in Fig. 54, except that a greater num- 

 ber of cells enter into the composition of the blastoderm, a 

 fact probably explained by the greater amount of yolk present 

 in the &gg of Ligia. Certain of the cells of the dark area, i.e., 

 according to my. interpretation the mesendodermal area, have 

 already sunk beneath the surface, and in the center of the area 

 is a distinct depression. Other material of the blastodermic 

 stage which I possess shows but little difference from this, the 

 cells in some of the ova examined being somewhat more 

 numerous, and between this condition and one in which the 

 embryo is distinctly outlined there is a gap. Nusbaum ('93) 

 has represented two later blastodermic stages in his Figs, i 

 and 2, showing in addition to a central thickening two lateral 

 ones, and assumes that the former gives rise to the endoderm 

 and the latter to the mesoderm. Sections which he gives, re- 

 peated in the abstract, show that in the region of the lateral 

 thickenings lower layer cells occur, and he believes that they 

 have been produced in situ. For this belief, however, it seems 

 to me he has given no conclusive proof, and it is quite possible 

 that we have to do here, as in other Isopods, with a migration 

 forwards of mesoderm cells, both the endoderm and mesoderm 

 differentiating from the central dark area of the blastoderm 

 and from, this alone. I do not assert that this is so, but the 

 fact of the close resemblance of the two stages I have figured 

 with what I have found in Porcellio and Armadillidiinn sug- 

 gests the supposition that all the processes of development of 

 Licria resemble those of Porcellio. 



4. General Consideration of the Segmentation. 



In the ova of the four species whose development has been 

 thoroughly studied we have to do with typical cases of centro- 

 lecithal or superficial segmentation, and certain facts have 

 been described which I believe have important bearing upon 



