82 Mr. A. Willey. On the [May 22, 



is, thirty-six hours after the commencement 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 Ampliioxus larvae 

 float in the midst of countless thousands of Sagitta larvae. 



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 I am not quite clear yet as to whether the last 

 two or three median slits ever move up to the right side. Meanwhile, 

 longitudinal ridges which are subsequently concerned in the forma- 

 tion of the atrium have appeared (see fig. 6). 



At the time of the completion of the atrium, which occurs at the 

 close of the larval period, some remarkable changes in the relative 

 position of parts of the body in the anterior region take place, by 

 which the mouth becomes median, and the gill-slits are arranged in 

 two series, a right and a left. The larva emerges from this critical 

 phase in its development as a symmetrical animal, but the details of 

 the process of symmetrisation the strongly-marked character of 

 which justifies the use of an otherwise undesirable term are still 

 rather obscure. The larva, now really a young Ampliioxus, with 

 atrium and paired gill-slits, ceases to lead a pelagic life, and takes to 

 the sand, where it passes the rest of its life. 



Spawning occurs at least from April to September inclusive. The 

 best month, however, in which to obtain the embryos is June, while 

 all the larval stages, up to the passage into the adult form, are to be 

 found during July and August. 



Previous View as to the Formation of the Atrium. 



The hitherto accepted method of formation of the atrial chamber 

 of Amphioxus is that described by Kowalewsky,* and more fully by 

 Eolph.f 



* ' Archiv fur Mikrost. !inat.,' vol. 13, 1877. 



t ' Morphol. JaLrbuch,' vol. 2, 1876. 



