DEVELOPMENT OF THE CAPE SPECIES OP PERIPATUS. 475 



nervous system ; this tissue, which is earlier and more largely 

 developed in the cerebral rudiments than in the ventral cords, 

 consists of a fine protoplasmic network, which stains but 

 slightly and is almost entirely without nuclei. A similar 

 network exists in the ventral nucleated parts of the nervous 

 system, but is there obscured by the crowded nuclei. At first 

 the whole central nervous system was similarly crowded with 

 nuclei (PI. XXXIV, figs. 15, 16 a, &c.), but in Stage d the 

 latter began to withdraw from the dorsal part, thus allowing 

 the non-staining protoplamsic network to stand out with great 

 distinctness (PI. XXXV, figs. 22, 23 a, &c.). At the boundary 

 between this white matter and the nuclear mass there are, in 

 Stages E, F, some nuclei which are somewhat larger than the 

 rest, and more loosely arranged, so that they may be said to 

 lie in the ventral part of the white matter (PI. XXXVI, 

 figs. 38, 39, &c.). Four such may generally be seen in each 

 section of the ventral cords. 



I must now pass to describe the changes of the more dorsal 

 parts of the lateral thickenings in the postoral region. In 

 Stage D they contain a single layer of oval nuclei, and become 

 at the same time pushed outwards by the outgrowths of the 

 hinder part of the mesoblastic somites (PI. XXXIV, fig. 9). 

 These outgrowths are arranged in pairs, one pair from each 

 pair of somites, and they constitute the rudiments of the 

 postoral appendages. The first pair to be formed is the pair 

 which will become the jaws; the next pair will form the oral 

 papillae, and so on in order from before backwards. When 

 the appendages are well established — in Stage e — it may be 

 seen from an inspection of transverse sections that they are 

 special developments of the posterior portion of lateral ridges 

 of ectoderm which extend on each side for the whole length of 

 a somite (PI. XXXV, figs. 18 a,b, 21 a, I. r.). Immediately 

 within these ridges there is a thickening of the somatic 

 mesoderm (m. t.) which I shall speak of again later, and 

 which is continuous with the thickened mesoderm of the 

 appendage itself. The postoral appendages, therefore, may be 

 described as special developments of longitudinal ridges, which, 



