THE EARLY DEVELOPMENT OF AMPHIOXUS. 611 



Fig. 13. — Transverse section of embryo of about ten hours; shows first 

 trace of collar cavity distinct from coelomic groove. 



Fig. 14, a, b, and c. — Three sections from a series through an embryo of 

 from ten to twelve hours in age. 



a. Shows the coelomic groove. 



b. Shows the coelomic groove closed off in front so as to form the 

 coelom. 



c. Shows the independent opening of the collar cavities. 



Fig. 15, a and b. — Two sections from a series cut parallel to the sagittal 

 longitudinal plane through an embryo of twelve to thirteen hours. 



a. Shows the coelomic groove and collar cavity opening into the archen- 

 teron. 



b. Shows neurenteric canal and division of trunk coelom into somites. 

 Fig. 16. — Transverse section of embryo of about thirteen hours, showing 



the left collar cavity only, opening into the archenteron. 



PLATE 44. 



(All the figures on this plate are drawn under the magnification of a 



Zeiss D, oc. 2.) 



Fig. 17, a, b, c, d, and e. — Five sections from a series through an embryo 

 of fourteen to fifteen hours. 



a. Shows head cavities opening into gut and anterior pore of the nervous 

 system. 



b. Shows head cavities constricted off from gut. 



c. Shows downward extension of right collar cavity and persistent com- 

 munication of the left with the gut {Neph.) (Hatschek's nepliridium). 



d. Shows the last somite, which is still continuous with the coelomic 

 groove. 



e. Shows the coelomic groove. 



Fig. 18, a, b. — Two sections from a series through an embryo of about 

 twenty hours. 



a. Shows the right and left head cavities becoming differentiated the one 

 from the other, the collar cavities having shifted forward. 



b. Shows that further back the two head cavities still communicate with 

 one another. The open neuropore is still seen. 



Fig. 19, a, b^ and c. — Three sections from a series through an embryo of 

 about twenty-four hours. 



a. Shows complete separation of right and left head cavities and the 

 ventral shift of the former. 



b. Shows inequality of the two collar cavities ; the left retains a narrow 

 communication with the gut {Neph.). 



