40 CHORDATE ORGANIZATION n. 10- 



which are indeed mostly formed of a single layer of ciliated epithelial 

 cells (Fig. 15). This is a striking indication of the lack of cephalization 

 of the animal. From the region of the cerebral vesicle spring the first 

 two dorsal roots, to which there are no corresponding ventrals. These 

 roots carry impulses from the receptors of the oral hood and its 

 tentacles. 



A B C 



Ifls jT^o*~r$5fc 



Fig. 16. Diagram to show the direction of the eye-spots of amphioxus. 

 A, anterior, B, middle, and c, posterior regions of the body. The eyes are shown as if seen from 

 behind. D shows the direction of spiralling of the animal when swimming — as seen from in front. 



(After Franz.) 



The infundibular organ (Fig. 15) is composed of tall cells with long 

 cilia, which beat in the opposite direction to those of the rest of the 

 vesicle. From them fibres run backwards down the cord. The organ 

 is also the site of origin of Reissner's fibre (Fig. 15). This is a thread 

 of non-cellular material, present in all vertebrates at the centre of the 

 neural canal. It is secreted at the front end and then passed backwards 

 and is often collected and absorbed in a sac at the hind end of the 

 spinal cord. In vertebrates it arises from secretory ependymal cells of 

 the subcommissural organ, lying dorsally in the diencephalon (Fig. 

 1 5). The infundibular organ of amphioxus is clearly not exactly similar, 

 yet the Reissner's fibres are clearly comparable ; an interesting problem 

 in homology. 



A further complication is that the cells of the infundibular organ 

 contain material that stains with the Gomori method, and is similar to 

 the neurosecretory material found in the fibres of the hypophysial tract 

 (Fig. 15). The organ thus seems to occupy a central position in the 

 control system as a receptor, originator of nerve-fibres, and of two 

 sorts of secretion. There is clearly much to be learned from this about 

 the origin and significance of the control systems of the diencephalon. 



In young stages the cerebral vesicle opens by an anterior neuro- 

 pore, and at the point where the closure takes place there develops a 

 depression of the skin, lined by special epithelium, and known as 

 Kolliker's pit. It is said to receive no special innervation. The cells 

 at the front end of the cerebral vesicle contain pigment and there have 



