THE NERVOUS SYSTEM AND THE VENTRAL ORGANS. 189 



The Nervous System and the Ventral Organs. 



The nervous system and the ventral organs arise from two massive 

 thickenings of the ectoderm formed by the active increase in number 

 of the cells on the ventral side of the cephalic and primary trunk- 

 segments. The two longitudinal swellings thus produced appear 

 at the time when the limbs become sharply marked off from the 

 body, and develop from before backward. 



According to Sedgwick, each of these swellings passes directly into a corre- 

 sponding thickening of the cephalic (antennal) segment, but this, according to 

 v. Kennel, is not the case, the swellings ending bluntly where the cephalic 

 section commences (Fig. 92), so that the part of the central nervous system 

 pertaining to the cephalic segment arises separately from the rest.* A much 

 slighter thickening of the ectoderm does, however, occur, according to 

 v. Kennel, between the cephalic and the trunk portions of the longitudinal 

 swellings at the time when the latter appear. And this, since it denotes the 

 formation of a commissure, might be regarded as indicative of a continuity 

 between the cephalic and the trunk portions of the longitudinal swellings. 

 This question as to the continuous origin of the cephalic and the trunk portions 

 of the central nervous system has already been discussed in connection with the 

 Annelida (Vol. i., p. 287). It was not indeed there finally settled, but it is in 

 connection with them that a decision of the question can best be expected. 



The paired thickening on the ventral surface just described gives 

 origin not only to the nervous system, but also to the ventral organs 

 (v. Kennel, No. 4). Transverse sections of the embryo show that 

 the thickening projects both externally and internally (Figs. 100, 

 p. 200, and 101, p. 202). In the middle of the cell-mass Avhich forms 

 it, a horizontal fissure then arises extending from before backwards 

 and separating the mass into an outer and an inner portion (Fig. 

 100 B). The inner mass of cells represents the rudiment of the 

 nervous system (»), the outer, remaining in connection with the 

 epidermis, represents that of the so-called ventral organs (vo), the 

 development and significance of which must now be discussed. 



The ventral organs. As the cleft, which in each segment divides 

 the rudiment of the nervous system from that of the corresponding 

 ventral organ, is interrupted by intersegmental cellular strands con- 

 necting its two walls (Fig. 101 B, p. 202), a segmentation of the 

 ventral organs takes place which is visible even externally. The 

 connecting strands between the ventral organ and the nerve-cord, 

 which occur between the consecutive pairs of limbs, are retained 



* An entirely distinct origin for the brain and for the ventral chain of ganglia 

 cannot here be asserted, inasmuch as the ganglion of the maxillary segment is 

 also drawn into the brain, as will be shown presently. On this account and also 

 because of the relation above pointed out, of the antennae to the limbs, there is 

 room for doubt as to the true cephalic nature of the brain in Peripatus. 



