THE ECTODERM. 57 



the central nervous system from the superficial ectoderm will 

 be described subsequently. 



It is only necessary for me to point out here that in the 

 region of the brain this separation does not occur, as the ecto- 

 derm is invaginated, and remains in connection with the 

 nervous tissue throughout life (PI. VII, fig. 22 a, and PI. VIII, 

 fig. 33). 



In Stage u a small amount of the so-called punctated tissue 

 appears on the dorsal surface of this rudimentary central 

 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 (Plate VI, 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 (Plate VII, 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 (Plate VIII, 

 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 (Plate VI, 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 ora» 

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

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



