28 OUTLINES OF CHORDATE DEVELOPMENT 



passage from the neurocoel to the archenteric cavity by way of 

 the blastoporal opening, is the neurenteric canal. This canal 

 remains open throughout the embryonic period, until after 

 the mouth opening is formed, so that this, in connection with 

 the neurocoel and neuropore, forms the only path by which the 

 archenteron is in connection with the exterior (Fig. 9, A, B, C). 

 The beating of the cilia lining the neurocoel probably keeps 

 up an interchange of fluids between enteron and the outside. 



Opposite the anterior margin of the first somite the medullary 

 tube is somewhat enlarged (Fig. 9, C), while opposite its 

 posterior margin, and, indeed, throughout the remainder of 

 the tube, it becomes somewhat narrowed by the elongation 

 of that part of the embryo, and a so-called brain region is 

 thus marked out. About the close of this period, pigment 

 spots begin to appear in the neural tube opposite the fifth 

 pair of somites (Fig. 9, C) : these are the first indications of the 

 development of sense organs and are doubtless photo-receptors. 

 The large cranial pigment spot in the anterior wall of the brain 

 appears about the close of the embryonic period. 



2. The Notochord 



The establishment of the chorda from the plate of endoderm 

 cells lying between the mesoderm folds, commences in the 

 region between the first and second somites (Fig. 8). Here 

 the plate becomes arched, its lateral halves folding together 

 ventrally. The folding extends posteriorly and anteriorly 

 from this region, and soon the folded plate is formed into a 

 solid strand lying below the neural tube, between the meso- 

 dermal somites (Fig. 8, E). Posteriorly the chorda extends 

 to the anterior wall of the neurenteric canal, and anteriorly 

 it slowly forms to the very extremity of the embryo, and 

 therefore in advance of the somites and neural tube (Fig. 9, 

 B, C). The extension of the chorda in front of the brain is a 

 peculiarity of Amphioxus, for in all other Chordates it forms 

 only to the region of the mid-brain. The chorda cells from 

 the two sides grow across the mid-line, interlocking and later 



