CEPHALOCHORDA I79 



which lie in the ependyma lining the central canal, the neurites of which 

 pass into the spinal cord (Edinger). This position of neuro-sensory 

 cells is explained by the primary origin of the spinal cord from an open 

 medullary plate lying on the superficial surface of the embryo, in the 

 same morphological situation as the neuro-genetic or neuro-sensory 

 epithelium of invertebrates. Similar cells have also been found in 

 Ptychodera (a species allied to Balanoglossus) by van der Horst, in the 

 collar region and extending forward and backward from this situation into 

 the dorsal region. 



Horn 



d.p.G 



I gtoug mrr^j 5 



r-n 



Fig. 129. — A : Lateral View of the Head-end of Amphioxus, showing the 

 Anterior Pigment Spot, the Dorsal Pigment Area of the Spinal Cord 

 and the Organs of Hesse. B : Section through one of the Organs 

 of Hesse. 



d.p.a. : dorsal pigment area of spinal cord. p. : 



H. org. : organs of Hesse. p.s. 



n. : nerve. r. : 

 N. ch. : notochord. 



pigment. 



: anterior pigment spot (eye). 

 rods. 



With regard to Joseph's statement concerning the histological 

 structure of the pigment-spot of Amphioxas, it must be remembered that 

 the cephalic region of this animal, which burrows head-downward in the 

 sand, is degenerate, and that notwithstanding the want of resemblance of 

 the " spot " to either a lateral or median vertebrate eye, the presence of 

 pigment in this situation is significant,' as pigment is one of the most 

 persistent of the tissue-elements both from the ontogenetic and phylo- 

 genetic standpoints. It is quite common in aborted and macerated human 

 embryos or foetuses to find the eyes represented merely by a mass of 

 degenerate pigment cells ; and Klinckowstroem has shown that the 

 position of the pineal organ in sea-gulls and other birds is represented by a 

 mass of irregularly distributed pigment cells in the parietal region (Chap. 3, 

 P- 75)- 



1 See p. 215. 



