ARTHROSTRACA AND CUMACBA. 137 



ingrowth of cells takes place, yields the elements of the entoderm 

 (cf. the cross sections, Fig. 68 A and B). The processes here revealed 

 are easily connected with the type of germ-layer formation found in 

 Astacus. The posterior unpaired formative zone of Ligia is com- 

 parable with the entodermal area (Fig. 60, ES) in Astacus, which 

 becomes invaginated later, and anteriorly to which is found the zone 

 of formation of the mesoderm (BM). The latter, in Astacus also, 

 according to Reichenbach, shows, even in early stages, a distinct 

 bilateral symmetry in the distribution of its elements. In Ligia, 

 these are found distributed in two paired areas of formation corre- 

 sponding with the two halves of the later germ-band. In what way 

 the formation of the actual germ-band out of these zones proceeds is 

 not known in detail. We may, however, assume that the elements 

 of the mesoderm shift anteriorly below the ectoderm, and that the 

 part of the ectoderm lying above them thickens. In any case, later 

 stages of Ligia and Cymothoa afford a distinct proof that the zone 

 of formation of the mesoderm just described must be considered as 

 answering to the posterior end of the future germ-band. For, in the 

 Nauplius stage of Ligia (Fig. 69), behind the third pair of limbs (3), 

 and situated anteriorly to the rudiment of the anal aperture (a), 

 there is found a formative zone (k) for the posterior end of the body 

 which develops in this region. This formative zone already consists 

 of two layers (ectoderm and mesoderm), the cells of which, by 

 division, have yielded a transverse row of large formative cells lying 

 posteriorly in front of the anal aperture (a) ; the arrangement of 

 these cells into transverse and longitudinal rows, both in the 

 ectoderm and the mesoderm, is exceedingly regular. According 

 to Patten (Quart. Journ. Micro. Sci., Vol. xxxi, 1890, p. 371), in 

 each of the transverse rows on either side, in Cymothoa, lie four 

 mesoderm-cells (Fig. 70, ms), by whose increase in number in the 

 anterior and more advanced segment-rudiments, the mesoderm of 

 the whole segment is developed, in such a way that the descendants 

 of the three lateral cells become connected, while the fourth more 

 medianly placed cell unites with the corresponding cell of the other 

 half of the body to form a common complex. This unusually regular 

 arrangement of the cell-material in the developing segments of the 

 posterior part of the body, recalls the very similar conditions observed 

 by Claus in the same region of the embryo of Branchipus. On the 

 other hand, the arrangement of the mesoderm-cells in a longitudinal 

 row running forwards through all the segments, calls for comparison 

 with conditions observed in the Oligochaeta by Wilson, and more 



