46 OUTLINES OF CHORD ATE DEVELOPMENT 



spreads out over what is by that time the partly formed oral 

 hood. It forms here the irregularly lobed ciliated patch of 

 cells called the " Rdder organ" (Figs. 12, 15, 16). The dorsal 

 portion of the preoral pit again divides into two parts, one 

 lying anteriorly and toward the right, the other posteriorly 

 and toward the left. The first of these remains on the right 

 side, in the roof of the buccal cavity, and forms there the 

 glandular "groove of Hatschek" leading to the oral aperture 

 (Figs. 14, 16). The second part remains toward the left side 

 and draws out into a long fine tube, extending back to the 

 pharynx, into which, by the time the larva has six gill slits, it 

 acquires an opening above the mouth, and loses its external 

 opening (Figs. 12, 15, 16). This part is sometimes called 

 " Hatschek's nephridium": its final structure is that of a greatly 

 enlarged nephridium (Fig. 16). There seems to be little doubt 

 that the dorsal division of the preoral pit is homologous with 

 the hypophysis of the Craniates, and with the neural gland and 

 its duct of the Tunicates. Some would restrict this homology 

 to the anterior portion (groove of Hatschek) alone, others to 

 the posterior part alone (Hatschek's nephridium). 



7. The Blood-vessels 



The ventral blood-vessel which was formed from cells of 

 somewhat uncertain origin between the lower ends of the 

 lateral plates as they approached the ventral side, is formed 

 throughout the pharyngeal region during the larval period, 

 lying between the primary and secondary gill slits (Fig 14, B). 

 It first begins to show contractions about the time the first 

 gill slit is perforated. 



8. The Atrium 



The presence of the atrium is one of the two chief character- 

 istics wherein Amphioxus is essentially unlike other Chordates, 

 for a similar structure is found only among the Urochorda. It 

 is to be recognized in Amphioxus as a special adaptation to 



