26 



Mr. W. Bateson. On the Development of [Dec. 18, 



mens (1 — 2 gills) a rod of hypoblast, solid in front, and behind con- 

 taining a lumen opening into that of the pharynx, is gradually 

 constricted off from the hypoblast in the dorsal middle line of the 

 pharynx. This hypoblastic rod grows forward into the proboscis cavity, 

 pushing in the mesoblastic lining. The lumen opens into the gut 

 throughout life. In B. Kowalevshii the opening is slit- like, and extends 

 through the middle third of the collar. A complicated structureless 

 skeleton is secreted by it on its ventral side, which is continued pos- 

 teriorly into two diverging horns, which lie in the walls of the fore- 

 gut. This skeletal structure in B. Kowalevshii differs slightly from 

 that described by Spengel for B. minutus. In young larvae the cells 

 of this hypoblastic rod are columnar, but they gradually become 

 irregular, vacuolated, and enclosed in a sheath, eventually in larvae 

 with seven or more gill-slits presenting an appearance precisely 

 comparable to that figured by Scott for the notochord of young 

 Lampreys, and by Balfour for that of young Elasmobranchs. It 

 becomes narrowed in the region of the proboscis stalk, where the 

 skeletal structures secreted by it attain their maximum thickness. 

 When isolated it is found to consist of hard cartilaginous tissue. It 

 serves to support the proboscis stalk, and to stiffen the wall of the 

 collar ; also as an attachment for the muscles of the collar and pro- 

 boscis. This is the statement of its function which is also made by 

 Spengel. 



From its development, position, histology, and function, it appears 

 to me to be comparable with the notochord of the Chordata, and by 

 this name it will be hereafter alluded to. This view of its homology 

 is supported by the presence in the Enteropneusta of many other struc- 

 tures pointing to vertebrate affinities. 



The Nervous System. — The commencing separation of the central 

 nervous system at the time of hatching as a solid cord in the dorsal 

 middle line of the collar extends itself anteriorly as far as the collar- 

 fold, and posteriorly to the region of the first gill-slit. In advanced 

 larvae (ten gill-slits) its anterior end contains a canal opening to the 

 exterior in the dorsal middle line at the anterior end of the collar- 

 fold. This canal w r ill be spoken of as the neural canal, and its pore 

 as the neural pore. As far as it is possible, in the absence of figures, 

 to follow Spengel's account of the development of this structure in 

 Tornaria, it appears to take place by the invagination of a median 

 furrow in this region, the neural canal being thus present from the 

 beginning. [As Spengel states, in B. minutus, &c, the lumen is 

 posteriorly not continuous, but is broken up by strands of nerve 

 cells.] This also holds for the anterior end in B. Kowalevshii. 



The rest of the nervous system, excepting that at the base of the 

 proboscis, does not appear till much later (three to four gill-slits). 

 Its position has been already described. Its origin in each case is due 



