120 DEVELOPMENT OF AMPHIOXUS. 



characteristic feature of the notochordal tissue throughout 

 the group of the Vertebrates. It is carried on to such an 



extent in Amphioxus as to 

 obscure the original cellular 

 structure of the notochord. 

 The cells anastomose with 

 one another in the longitu- 

 dinal direction, and so pro- 

 duce a reticulum the meshes 

 Fig. 69. Median sagittal section of of which represent the vacu- 



notochord of a young Amphioxus of o]es whose firgt { j 

 8 mm., to show the vacuolar character 



of the notochordal tissue and the dis- shown in Fig. 66. Most of 

 placement of the nuclei to the dorsal and . 



ventral borders. (After LWOFF.) the nuclei become eventually 



displaced from the centre of 



the notochord, and are, in the adult, almost exclusively 

 confined to its dorsal and ventral aspects (Fig. 69). 



The Prcsoral "Head-cavities" of Amphioxus. 



Before leaving the embryonic period of the development 

 it is necessary to consider the origin and fate of what may 

 be called the head-cavities of Amphioxus as made known 

 to us by the work of Hatschek. 



They arise symmetrically as a pair of diverticula from 

 the anterior portion of the archenteron, which lies at first 

 partly in front of the notochord (Fig. 68 bis) and completely 

 in front of the myoccelomic pouches (Fig. 70). 



They begin to appear at the stage in which some eight 

 pairs of pouches are already present. Their origin there- 

 fore, in point of time and the subsequent modifications 

 which they undergo, show that they do not belong to the 

 metameric series of the mesodermic pouches, but are 

 structures sui generis. 



