278 



COMPARATIVE ANATOMY 



CHAP. 



npr 



npr 



lib 



Ventral Chord of the Annulata. The ventral chord seems always to begin to 

 form separately from the neural plate. It arises either as a continuous thickening 

 of the ectoderm in the Ventral middlp line, or as a pair of thickenings one on each side 

 of this middle line. The differentiation of the rudiment of the ventral chord into 

 the definitive ventral chord goes hand in hand with the development of the rest of 

 the trunk, and proceeds from before backward. It either remains, like the brain, 

 connected during life with the ectoderm, or it becomes constricted off from it and 

 takes up a position either in the musculature of the body wall or still deeper in the 

 body cavity. At the posterior end of the body it almost always retains its embry- 

 onic condition, as it here remains throughout life in its place of formation, the 

 integument. The Hirudinea and Lumbricus among the Oligochceta differ very 

 much from other Annulata and Sipunculidce. The ventral chord here does not arise 

 in situ in the ectodermal integument ; but two ectodermal segmentation spheres 

 (micromeres) appear veiy early near the posterior end, and take up a position under 

 the ectoderm, lying symmetrically on each side near the middle line. New cells are 



continually constricted off anteriorly from 

 these neuroblasts (Fig. 188, nb), which 

 again divide, and a cell strand thus arises 

 on each side of the ventral middle line, 

 immediately beneath the integument. The 

 two cell strands, which form part of the 

 germ streaks of the Hirudinea and of Lum- 

 bricus, represent the rudiment of the ventral 

 chord, which, beginning behind the mouth, 

 becomes continuously differentiated from 

 before backward. It is evident from this 

 that the rudiment of the ventral chord is 

 unusually localised, and at the same time 

 is to be referred to a very early stage of 

 development. The connection of the ventral 

 chord with the brain through the cesophageal 

 commissure seems everywhere to take place 

 secondarily. It may perhaps in time be 

 proved that the central nervous system in 

 the Worms and Platodes proceeds onto- 



streaks in a Lumbricus embryo (after Wil- genetically and phylogenetically from two 

 son), pme, Pole cells of the mesoderm (ineso- cMef parts> viz< first from the sen sory part 

 blasts); n* pole cells of the ventral chord ^ ^ united 



(neuroblasts) ; npb, pole cells of the nephnclial 



rows (nephroblasts) ; x, pole cells of the cell centres or sensory ganglia of the anterior 

 rows (xr) of unknown significance ; ec, ecto- end of the body, and second from the motor 

 derm ; ez, large ectoderm cells ; npr, nephridial cen tral nervous system, i.e. the ventral 

 cell rows ; nr, neural cell rows ; m, mesoblast chordj ^ 3^^^ commissure, and the 



motor part of the brain of the Annulata, 



the longitudinal trunks and the motor part of the brain of the other worms and 

 Platodes. In the Nemertina, however, the lateral nerves are said to grow out from 

 the brain posteriorly. This may perhaps here, and also in the Turbellaria, point 

 to a concentration of the whole central nervous system into one single rudiment. 



In the development of the other component parts of the mesoderm, we find that, 

 just as the rudiment of the ventral chord in the Hirudinea and Lumbricus is shifted 

 back to an early stage, and is condensed into two germ cells (the neuroblasts), so the 

 rudiment of all other mesodermal organs in worms are extremely condensed and 

 localised and shifted back to early stages, so that generally a few germ cells or a 



FIG. 188. Superficial aspect of the germ 



