376 ADAM SEDGWICK. 



lateral nerve-cords (No. 2, fig. 14), and these two connections, 

 together with the interposed ventral organ, contribute what 

 Balfour has called the second commissure between the ventral 

 cords. 



The posterior part of this ventral organ behaves exactly as 

 do those about to be described. 



4. The two halves, of which each ventral organ of 

 the seventeen ambulatory legs at first consist, join one 

 another ventrally, remain as part of the ectoderm, and appear 

 to retain a cellular connection with the lateral nerve-cords 

 (PL XXVI, fig. 5). I could not be certain of this connection 

 in the case of every ventral organ ; but Kennel asserts that it 

 exists, and I am inclined to agree with him. 



Nervous System. 



The early development of the central nervous system is 

 described in Part III, pp. 473 — 475 and 481. In Stage f the 

 cerebral grooves are still open (Part III, fig. 33), and the 

 ventral cords are in close contact and still continuous with the 

 thickened ventral ectoderm. The white matter has also made 

 its appearance along the whole length of the dorsal side of 

 both brain and spinal cord. 



In Stage g two important changes have taken place. 

 (1) The cerebral grooves have become converted into closed 

 vesicles (PI. XXVI, figs. 2 and 3) and entirely cut off from the 

 superficial ectoderm. (2) The ventral cords have withdrawn 

 themselves from the ventral ectoderm, though they still appear 

 to be attached to the latter by marked cellular processes (fig. 5, 

 PI. XXVI). 



The Brain. — The structure of the brain in Stage g is shown 

 by the series of sections figured (figs. 1 — 4, PI. XXVI). Ex- 

 cepting the increase in the amount of white matter and the 

 closure of the cerebral grooves it is essentially the same as in 

 Stage f. In front the two lobes of the brain, though in close 

 contact, are separate from one another (PI. XXVI, fig. 1). 

 They are continued forwards into the antennas as the anten- 

 nary nerves (vide Part III, p. 480). The white matter is 



