564 CEPHALOCHORDA. 



medullary canal (aquaeductus Sylvii, II). In the third part (the 

 hind-brain) the central canal shows a narrow ventral tube, while its 

 dorsal portion is independently dilated so as to form a vesicle covered 

 by a thin membrane, the fossa rhomboidalis of HATSCHEK (III). 



The medullary tube originally opens externally through a neuropore 

 bordered by closely crowded flagella. This is pressed out of the 

 middle line to the left through the development of the dorsal fin. 

 An epithelial depression forms later at this point ; this is KOLLIKER'S 

 olfactory or ciliated pit (Fig. 298, rc), at the base of which the neuro- 

 pore opens. The ciliated pit is the homologue of the olfactory organ 

 and of the hypophysis of the Craniata. The short, unpaired olfactory 

 nerve discovered by LANGERHANS runs from the brain to the posterior 

 wall of the ciliated pit. The continuation of the first ventricle towards 

 the neuropore corresponds, as above mentioned, to the infundibulum 

 of the Craniata. In Ammocoetes, the hypophysis and the olfactory 

 organ are still connected together, and the external aperture of the 

 hypophysis still retains its original dorsal position. The inner end of 

 the hypophysis, on the contrary, and the infundibulum have moved 

 to the ventral surface of the brain. 



In identifying the anterior end of the medullary tube of Amphioxus with 

 the infundibulum of the Craniata we have followed HATSCHEK. It should, 

 however, be mentioned that KUPFFEB has recently been led by his researches 

 on Acipenser (No. 38) to homologise the anterior end of the cranial axis of 

 Amphioxus with his lobus olfactorius impar which lies near the lamina 

 terminalis above the anterior commissure, and indicates the point where the 

 medullary tube remained longest in direct connection with the ectoderm 

 precisely as in Amphioxus, and to regard the infundibulum as a secondary out- 

 growth of the ventral side of the brain. 



It should here be pointed out that KOHL (No. 9) occasionally noticed, on 

 the right side of the head of Amphioxus, a pit resembling the olfactory pit of 

 the left side. KOHL is inclined to consider this as the half of an originally 

 paired olfactory organ in a vestigial condition. The relations of the neuropore 

 to the olfactory pit were denied by KOHL. The remains of the neuropore are 

 said to be found in an ectodermal depression lying somewhat behind the 

 olfactory pit. 



Our knowledge of the important transformations that take place in 

 the primitive segments and the body-cavity is due chiefly to HATSCHEK 

 (No. 7). Each primitive segment becomes divided by a transverse 

 partition into two portions, as has already been stated (p. 547), viz., 

 a dorsal portion (proto- vertebra) and a ventral part (lateral plate, 

 Fig. 300, / and //). Only in the dorsal portion is the primitive 

 segmentation retained, the adjacent walls of the segments persisting 

 as the transverse septa (myosepta). In the region of the lateral 



