CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 29 



those in the Annelida, from which by repeated division the mesoderm- 

 banda are formed. In this way, the bilateral symmetry of the body 

 would find an early expression in the rudiment of the mesoderm. 



Two mesoderm-bands, which, however, are not nearly so regular in their 

 arrangement as in the Annelida, have also been discovered by Rabl and 

 Hatschek. Horst has described similar conditions in Ostrea, and Ziegler's 

 account of Cj/clas also, on the whole, agrees with the above. The latter 

 author, however, does not exclude the idea of a further participation of the 

 ectoderm in yielding the mesoderm-elements, and Ray Lankester also was 

 formerly in favour of the partial derivation of the mesoderm from the ecto- 

 derm (Pisidivm). There was therefore a general inclination not to derive the 

 whole of the mesoderm in the Lamellibranchia from the primitive mesoderm- 

 cells. 



I'nio, a form in which the mesoderm and the germ-layers in general were 

 first demonstrated by Rabl, although indeed not very accurately (c/. pp. 

 27 and 50), shows most markedly the method of formation of the primitive 

 mesoderm-cells [mesodermal teloblasts] and the mesoderm-bands. But since 

 the entodermal nature of the large invagination in the Unionidac must be 

 considered as refuted, these conditions also cannot be regarded as sufficiently 

 established. Rabl found, in Unio, two cells which even in the blastula-stage 

 are distinguished by their size from the rest. At the commencement of 

 gastrulation, these pass into the cleavage-cavity, and then lie symmetrically 

 to the median line. The active multiplication of these two cells is said to 

 give origin to the mesoderm-bands. 



It must here be mentioned that the presence of the large cells that were 

 found by Rabl within the young embryo is confirmed by the later descriptions 

 of Schierholz (No. 49) and Goette (No. 15) (Fig. 23 -4, p. 51). According 

 to Goette's figures, these might also lie near the blastopore, since the latter 

 is apparently not far removed from the shell-invagination which Rabl mis- 

 took for the archenteron (Fig. 23 .4). The mesoderm-bands in the Lamelli- 

 branchia cannot, as a rule, be said to be very distinct. 



[The primitive mesoderm-cells in Unio lie in the cleavage-cavity imme- 

 diately posterior to the blastopore, and give rise to two mesoderm-bands by 

 teloblastic growth. There are in addition certain mesoblastic cells (the larval 

 mesoblast of Lillie) situated anteriorly to the archenteron, which have the 

 character of a mesenchyme, and possibly form the larval adductor muscle and 

 the myocytes.] 



Summary. The differentiation of the germ-layers in the Lamelli- 

 branchia takes place very early. Even during cleavage the two 

 primary layers can be clearly distinguished, and the rudiment of 

 the middle germ-layer can also be recognised very early (Figs. 

 12-14). After the gastrula-stage is reached, the mesoderm is found 

 in the form of more or less massive accumulations of cells (mesoderm- 

 bands), apparently proceeding from the posterior pole, between the 

 ectoderm and the entoderm. The bilateral symmetry of the germ 



