ON THE DEVELOPMENT OF LINGULA. ANATINA. 69 



compared to a hollow hemisphere compressed dorso-ventrally. 

 The middle part is thickest, and from this region it thins out 

 gradually in every direction. Its ventral edge is prolonged further 

 posteriorly than the dorsal. The ganglion is not an eminence 

 projecting inward into the body cavity from the anterior body 

 wall as a posteriorly directed thickening, as Brooks figures in 

 his well known diagramatic sagittal section of a Lingula larva 

 (PI. VI., Fig. 1(3), but it is a thickening directed and projecting 

 anteriorly. The lateral ganglia at this stage extend for a little 

 distance forward along the neck. 



As to the histogenesis. As is seen in a longitudinal section 

 of a larva at the 6 p. c. stage (PI. VII., Fig. 114), the epi- 

 thelium, which is to become the ganglion, at first increases in 

 thickness and the nuclei, each with one nucleolus, acquire a 

 vesicular character. The nuclei then travel peripherally, the cyto- 

 plasm being drawn out gradually taking the form of nerve fibres. 

 At the 7-9 p. c. stage (PI. VIL, Fig. 10<3) the fibrous layer 

 increases in thickness and the nuclei come to occupy a thin 

 peripheral layer. The fibres in frontal as well as in transverse 

 sections present the typical granular appearance peculiar to a 

 section of fibres, while in a sagittal section almost the whole 

 course of the fibres is seen. It can thus be determined that 

 a majority of the fibres run dorso-ventrally. In the nuclear 

 layer placed peripheral to the fibrous one in the ganglion a 

 differentiation has already taken place at this stage : the distal 

 nuclei epithelially arranged in one row have become rather com- 

 pact, acquiring a stronger affinity for nuclear stains ; a layer of 

 nuclei (3-4 thick) proximally placed, on the other hand, retains 

 its original vesicular character. The former is composed of the 

 ordinary epithelial cells while the latter of ganglion cells (PI. 



