155]' LIFE HISTORY OF GORDIUS AND PARAGORDIUS—MA Y 35 



The hypoderm is always of greater diameter at the ends of the body 

 than in the middle. The elongated cells forming the postcloacal ridge of 

 the male have already been mentioned. Other modifications of the hypo- 

 derm will be taken up in the next topic. 



Nervous system. The nervous system consists of brain, ventral cord, 

 cloacal ganglion, peripheral fibers and nerve cells located in various parts 

 of the hypoderm. 



Central nervous system. The very early appearance of the central 

 nervous system has already been described. In the description of the later 

 development each part will be taken up separately. 



As stated in a previous section, the brain is outlined at first as a ring of 

 cells in the hypoderm of the proboscis. It soon separates from the hypo- 

 derm, remaining connected with it only at the anterior end and the ventral 

 side (Figs. 57, 67-69). It consists at this time of a few large cells situated 

 just in front of the partition between the region developed from the larval 

 proboscis and that developed from the body of the larva. The cells com- 

 pletely surround the larval muscles and the strand that connects the sti- 

 lets with the partition in the larva. These large cells remain permanently 

 in that position (Figs. 73, 74) while the rest of the brain develops in front 

 and around them, most of the later cells appearing antero-ventrad to the 

 original group. By the growth of the anterior region the larval connecting 

 strand becomes stretched out and torn, the major part of it usually remain- 

 ing in the base of the brain, while the armature of the proboscis is carried 

 forward and pushed out of the hypoderm at the anterior end (Figs. 73, 

 22, 23). The strand in the base of the brain later disintegrates, leaving an 

 open space (Fig. 74). At the time the cells have reached their full develop- 

 ment the original group forms the postero-dorsal part of the brain (Figs. 

 81-83), while the other cells, mostly smaller in size, surround the rest of 

 the brain and form a heavy mass at the ventral side continuous with the 

 cells of the ventral cord (Figs. 73, 81-83). The central core of the brain 

 is occupied chiefly by fibres with a few scattered cells. From the ventral 

 cell mass a group of cells projects dorsad into the anterior part of the fibre 

 mass and tends to become separated by intervening fibres from the 

 underlying cells. Ventral and slightly posterior to this group of cells 

 is a large, definitely outlined cross commissure (Fig. 74) dividing at each 

 end into a ventral and a dorsal branch. The cells of the brain appear 

 to be multipolar but that fact has not been definitely established. 



The fibres in the brain pass in all conceivable directions, and many 

 of them are directly continuous with those of the cord. At the anterior 

 end under the white area and part of the dark ring the hypoderm is 

 very much thickened, most of the cells are modified into bipolar nerve 

 cells like those which connect the ventral cord to the hypoderm in the rest 

 of the body, and on the ventral side these fibres pass directly over into 

 the connecting fibres of the cord. 



