DEVELOPMENT OF ATRIAL CHAMBER OF AMPHIOXDS. 447 



hours of seven and eight — and goes on very rapidly through 

 the night. 



The early stages have been so fully described by Hatschek ^ 

 that it will only be necessary to refer to them in the briefest 

 manner. 



At 8 p.m. segmentation commences; at 11 p.m. invagination 

 commences; at 1 a.m. the gastrula is complete; at 3 a.m. the 

 gastrula begins to revolve by cilia within the yolk-membrane; 

 and at 5 a.m. two pairs of myocoelomic pouches have been 

 formed, and the embryo ruptures the egg-membrane and 

 becomes free-swimming. 



During the first day the embryo grows in length and adds 

 several pairs of somites. By about eight o'clock on the 

 second morning — that is, thirty-six hours after the commence- 

 ment of segmentation — the embryo has acquired a mouth on 

 the left side of the body, and a gill-slit, which arises at first in 

 the median ventral line, and subsequently comes to lie on the 

 right side of the body. 



The anus is formed soon after the appearance of the mouth 

 and first gill-slit. 



The embryonic period is now at an end, and the larval 

 period begins. As Hatschek states, the only way of obtaining 

 the larval stages is by pelagic fishing. This consists in 

 dredging at depths varying from fifteen to twenty fathoms. 

 At this depth the Amphioxus larvse float in the midst of 

 countless thousands of Sagitta larvse. 



A long, but not yet clearly ascertained interval (probably 

 about a fortnight) elapses between the formation of the first 

 and second gill-clefts. 



In the period during which it is free-swimming the larva 

 acquires from twelve to fifteen consecutive unpaired gill-slits, 

 each one arising in the mid-ventral line, and then growing in 

 such a manner as to lie on the right side of the body. This 

 applies to the anterior two-thirds of the pharynx, but it is not 

 quite clear yet as to whether the last two or three median slits 

 ever move up to the right side. Meanwhile, longitudinal 



1 Claus's 'Arbeiteii,' ISSl. 



