The Gill-Slits . The gill-slits are not formed un- 

 til after the peribranchial cavity has been completely sepa- 

 rated from the branchial sac. The first to appear lie far 

 back towards the posterior ' end, but very soon they begin to 

 break out in spots all over the sides of the branchial sac. 



The tendency to arise in vertical rov/s becones ap- 

 parent, when only very fev/ are present, but eacii slit is a 

 separate and independent formation. I have never observed 

 the origin of one slit from another, such as occurs in the 

 larvae of Ascidians. 



Pig. 5, represents a stage when about eight slits 

 have been formed on each side; as the anterior pouches of 

 the peribranchial sacs grow further and further fon.vard, new 

 rov/s of slits are laid down along their free margins. 



The first indication of a gil]-slit is a small cir- 

 cular, thickened area of the branchial wall, which at this 

 spot becomes slightly evaginated until it touches the visce- 

 ral wall of the peribranchial cavity^ ^ig. 22, a. The 

 cells of the latter at this point become thickened somewhat, 

 and now a fusion takes place between the two v;alls; this is 

 seen in the lower portion of Pig. 22, b. The opening, which 

 breaks through the centre of the fused patch of cells, is 

 drawn out later in the long axis of the bud into a narrow 

 slit, which is provided v/ith cilia in the usual way. The 

 upper part of Pig. 22, b^, shows a slit just after the opening 

 has been formed. 



The Branchial and Cloa cal Orifices . The branchial 

 orifice arises at a tolerably late stage, and is first indi- 



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