lateral sacs, surrounding the spacious branchial sac, and 

 connected dorsally bj'^ a median space, the cloacal cavity. 

 The lateral sacs are unsymmetrical, however, until quite a 

 late stage, for the anterior pouch of the right peribranchi- 

 al sac grov/s more rapidly and extends further forward than 

 the sirrilar pouch on the left side, while the posterior pouch 

 of the left side extends further backv/ard than the corres- 

 ponding one on the right side. Eventually the two sacs be- 

 come symraetrical. 



The formation of the peribranchial cavity is eas- 

 ily understood from the series of sections represented in 

 Pigs. 13--1S; these v/ill be rendered more intelligible by a 

 comparison with Pig. 4, which is drawn from a total prepara- 

 tion of a bud at the same stage of development. The sec- 

 tions are taken respectively at the levels indicated by the 

 parallel lines, a_, _b, _c, d, e^ and f_ of Pig. 4. In Pig. 13, 

 line a of Pig. 4, the most anterior one of the series, the 

 branchial sac, br, is seen by itself, for the extensions of 

 the peribranchial sacs have liot re.ucle-ct far enoi.'gii forv/tird to 

 appear in the section; the hypophyseal tube, d . t , is shoivn 

 on the dorsal side of the branchial sac. Fig. 14, line _b, 

 only includes the anterior extension of the right sac, for, 

 as just stated above, the pouch on the opposite side lags be- 

 hind, in its growth. Fig. 15, line c, represents a section 

 taken just in front of trie anterior face of of the cloacal 

 cavity, and shov/s both peribranchial sacs at the level where 

 they are continued foTtfard into their anterior extensions. 



