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



increase in the number of mes-endoderm cells as well as of the 

 surrounding ectoderm cells, the latter increase being due 

 mainly, if not entirely, to the division of the cells which have 

 already undergone concentration in Fig. 50, there being prob- 

 ably no further migration of cells from the dorsal surface of 

 the Q.^^ to form the blastoderm. In these last two figures a 

 row of four cells {Me) not at all well-marked off, however, lies 

 at the anterior margin of the mes-endoderm zone, and it is pos- 

 sible that they correspond to the row of mesoderm cells occur- 

 ring in Aselhis, though it has been impossible to distinguish them 

 in later stages, and accordingly their significance is uncertain. 



An Q^g of Porcellio, in a slightly later stage than Fig. 50 

 though earlier than Fig. 51, is represented in Fig. 53. From 

 this it will be seen that the process of development proceeds 

 in this species as in Armadillidhim. There is a similar contin- 

 uation of the concentration of ectoderm cells around the mes- 

 endoderm, to form the blastoderm, and in Fig. 54 a later stage 

 is shown, in which the cells of the ectodermal portion of the 

 blastoderm are considerably increased, and in which the con- 

 centration is somewhat greater. In the preparation from 

 which this drawing was made the blastoderm had been dis- 

 sected from the surface of the yolk, and in the operation was 

 slightly torn ; enough of the yolk was left adherent, however, 

 to support a few of the scattered ectoderm cells which do not 

 take part in the formation of the blastoderm. Up to about 

 this stage in Porcellio^ and in Arinadillidm7n also, all the cells 

 are at the surface of the yolk, but now careful focusing of the 

 mes-endoderm region shows that there are a number of cells 

 below the surface, a fact which is also shown by sections of 

 the blastoderm (Fig. 55). I found no indication of spindles 

 perpendicular to the surface in my preparations of this stage, 

 and am inclined to believe that the cells of the lower layer 

 arise by immigration rather than delamination, although I am 

 not in a position to maintain that the latter process does not 

 occur. It is certain, however, that at this stage lower layer 

 cells occur only in the mes-endoderm region. 



In Fig. 56 a somewhat older blastoderm of Porcellio is shown, 

 s'r.i'l.ir pr2prj"?.t'ons of ArniadiUidittm having also been ob- 



