NO. 3 



INSECT HEAD SNODGRASS 23 



a prostomial region and of six postoral metameres with paired coelomic 

 sacs, of which the first metamere bears a pair of small, evanescent 

 preantennal appendages (fig. 14 B, Pnit). Wiesmann, however, claims 

 that the prostomium is a segment, because he finds in its mesoblastic 

 tissue a pair of small cavities at the base of the paired rudiments of the 

 labrum. The prostomial region of the adult arthropod contains a part 

 of the body lumen, but from this it does not necessarily follow that its 

 primitive mesoblastic cavities are homologous with the coelomic sacs 

 of the true mesoderm, the extent of which should be limited by the 

 length of the blastopore (see page 16). More likely, the mesoblast of 

 the prostomium is a mesenchyme. In any case, however, it is only a 

 matter of definition as to what we shall call a " segment." 



The assumption of the presence of one or more preoral segments 

 in addition to the prostomium disregards the fundamental relation 

 between the embryonic germ layers. As already pointed out, the 

 position of the mouth, or of the stomodeal invagination, marks the 

 anterior end of the blastopore ; the extent of the endoderm, except as 

 it expands within the body, is determined by the length of the blas- 

 topore; the mesoderm is derived from the endoderm, and in the 

 mesoderm metamerism originates. Therefore, in a bilateral animal, 

 it seems clear, true segments can not lie morphologically anterior to 

 the mouth. In the insect embryo, the anterior mesenteron rudiment 

 actually defines the anterior limit of the mesoderm. Later formed 

 segmental regions or appendages that appear to be preoral must, then, 

 have acquired this position secondarily. In the figure of a Peripatus 

 embryo (fig. 4 D) it is clearly seen how the anterior coelomic sacs 

 may extend laterally before the mouth, and how corresponding 

 appendages might come to have a preoral location topographically, 

 though being morphologically postoral. 



In the insect brain, there has never been noted a distinction between 

 ganglionic rudiments of a preantennal segment and prostomial ele- 

 ments in the composition of the definitive protocerebral lobes, and 

 the optic lobes are commonly referred to the first segment, though 

 their independent origin is recognized. In the Crustacea, however, pre- 

 antennal ganglia have been recorded, and Daiber (1921) says, " since 

 ontogeny appears to give support to the view that the optic lobes are 

 secondary structures, we must suppose that the segmental gangha of 

 the preantennal segment have been mostly suppressed, and that remains 

 of them are represented in the ganglion cells of the roots of the oculo- 

 motor nerves. The ganglion pair found in the embryo of Astacus and 

 lacra between the ganglionic fundaments of the optic lobes and those 



