128 CHANGES FROM STAGE G TO BJUTH. 



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

 to retain a cellular connection with the lateral nerve-cords 

 (PI. Xj 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 Ch. Ill, pp. 56 — 58 and 64. In Stage f the 

 cerebral grooves are still open (PI. VIII, 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. X, 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 (2) The ventral cords have withdrawn 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. X). 



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

 shown by the series of sections figured (figs. 1 — 4, PI. X). 

 Excepting 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. X, fig. 1). 

 They are continued forwards into the antennae as the anten- 

 nary nerves (vide Ch. Ill, p. 62). The white matter is 

 dorsal and extends somewhat into the centre of the lobes. It 

 may be described (vide Balfour, No. 20) as consisting of three 

 horns: viz. a dorsolateral (a), a ventro-lateral {b), and a dor- 

 sal (e). It is continued forwards along the dorsal sides of the 

 tentacular nerves. 



At a little distance behind the eyes the central lobe of white 



