20 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvii. 



finally reach the periphery, when segmentation first begins to appear 

 on the .surface. The egg now becomes a syncytial blastula, the cavity 

 being completely filled with yolk. At this stage the histological difler- 

 cntiation of cells is very complete, although it is indicated in the pre- 

 ceding stage. In J»^ra, four cells, termed the vitellophags, occupy 

 the posterior pole. Surrounding them is a circle of twelve cells, the 

 mes-endoderm cells. The sixteen ectoderm cells are scattered over the 

 remainder of the surface of the egg. In the next stage, the sixty-four- 

 celled stage, the vitellophags have increased to eight in number, the 

 mesoderm cells forming a double ring around them, each ring consisting 

 of twelve cells, and the number of ectoderm cells is doubled. In the 

 succeeding stage the vitellophag cells do not take part in the division. 

 One cell of the posterior circle of mes-endoderm cells divides in a dif- 

 ferent plane from the others, and one of the daughter cells probably 

 gives rise to the endoderm which forms the liver lobes. In Asellus 

 the differentiation of vitellophags from the mes-endoderm is distinct at 

 first but becomes inconspicuous later, and the differentiation of the 

 liver endoderm is questionable. In ArmadilUdlum. and Porcellio no 

 differentiation of the three parts of the mes-endoderm is recognizable, 

 although the mes-endoderm in these forms is equivalent to the mes- 

 oderm, the liver endoderm, and the vitellophags as found in Jcvra. 



In the next stage the cells of the mesoderm, endoderm, and ecto- 

 derm increase in number and migrate to the ventral surface where 

 the embryo is to be formed. The concentration of the mesoderm cells 

 results in the formation of a mesoderm plug, which becomes gradually 

 covered by ectoderm cells, the teloblasts, arising from the posterior 

 row of ectoderm cells, and arranged in regular longitudinal and trans- 

 verse rows. When the mesoderm plug is about half covered with 

 ectodermal teloblasts, the vitellophags in Jxra begin to migrate into 

 the interior of the yolk. The ectoderm cells of the anterior portion 

 of the body, together with the mesoderm cells, contained in the meso- 

 dermal plug, which has gradually migrated and become distributed in 

 that region, represent the naupliar part of the embryo. The liver 

 endoderm migrates also to this region with the mesoderm. The pos- 

 terior or metanaupliar region lies behind it and has resulted by the 

 growth of the teloV)lasts. 



The mesoderm cells in the naupliar region, lying beneath the ecto- 

 derm cells, become arranged in two divergent bands. In r/rt^/'« a trans- 

 verse band joins the anterior ends of the two lateral bands, but this is 

 not distinct in the other forms. As the appendages bud out the meso- 

 derm cells migrate into their interior, forming a solid support for 

 them. The liver lobes, formed from the liver endoderm, l)egin to 

 appear about this time as hollow spheres, open toward the 3"olk, one 

 on either side at the level of the first maxillte. 



The metanaupliar mesoderm is produced by teloblastic growth. The 

 mesoblast cells become arranged in a very definite manner in transverse 



