10 



THE ANCESTRY OF VERTEBRATES 



from the exterior into tiie archenteron. While all other 

 developmental processes in the segmented animals proceed 

 from in front backwards, the formation of the medullary 

 tube in lower Chordates has its starting point at the blas- 

 topore and proceeds in a forward direction. Characteristic 

 of the stomodaeum, at least in young stages, is the dense 

 coat of cilia which induces a current of water. In Am- 

 phioxus the neural tube is also clothed with cilia, equally 

 producing a water current from in front backwards (HAt- 

 SCHEK, 1882, p. 59, 71) and in higher Chordates a fine coat 



Fig. 3. (cf. fig. 11). Diagram representing the derivation of a 

 Cliordate (Acraniate) from an Annelid (the Annelid is placed 

 with the ventral side up). 

 Can. med. medullary canal, m. mouth, /7eur. /?. n euro pore, 

 p. card, cardiac pore, p. near, neurenteric pore, stom. stomo- 

 daeum. 1, 2, 3, mesoderm segments. 



The endoderm has a darker colour than the ectoderm. 



of cilia is often observable covering the ependyme of the 

 neural tube or the neural plate. Thus MORGAN (1890) remarks: 

 "Mr. Wightman has demonstrated to me in the neural tube 

 of adult frogs the ciliated epithelium in the living condition, 

 and further by the addition of suspended carmine granules 

 these cilia are seen to drive the particles towards the tail." 

 The agreement between the stomodaeum and the embryonal 

 medullary tube is almost complete if only we assume that 

 in Chordates the former has become very long and has- 

 extended over the whole length of the body, carrying the 

 cardiac pore backwards to the hinder end where we find 

 it now as the neurenteric canal. In the last chapter we 



