priman" slits. This new slii consiitu:es the deliniti\-e f.rst 

 sill of the seconcian" series (Fig;. ~~\ 



The ian"a shown in Fig. 77 presents a ven" different 

 .^^rect from that shown in Fig. '4: the transition from one 

 stage to the other is. of course, gradual, and all intermediate 

 strvs car. be obsen'ed. In the stage which we are now 

 considering (Fig. 771. the atrial cavitv has become com- 

 pletely closed up in front, so that now none of the gill-slits 

 open directlv to the exterior. 



Xone of the priman" slits now lie entirelv on the right 

 si,":e. but thev have become bent under the phar^"nx. and 



Fi£. 77. — .Arj'.enor pcmcD o: '.^tta, in A^-hich the second^iy slits h^ive become 

 perfcTaiec, ^Xid the pnmarr s'.i:s are passing round to the ieit side. From the rii:ht 

 sde. lAiter WiLLEY.^i 



j^'. Sense-o-g-an of pr.ieorai pit. :. Righ: half of velum. £, EndosKle. grow- 

 inc bevoni the club-sha.ped gland ^fl. /^f . Firs: primary slit, much reduced in 

 =:re. sj . First secondarv slit. /_f^. Twelfth piimar}- slit, behind which is to be 

 seen a Tesrlce oi the thirteenth siii- 



thus extend round to the left side. This bodily migration 

 of the primary slits from one side to the other occurs in 

 correlation with the increase in size of the secondai'y slits, 

 which, as they continue to grow, push, as it were, the 

 primary- slits before them, and so cause the latter to bend 

 under the pharvnx in the way described. The peculiar 

 growth bv which the primar\- gill-slits are gradually carried 

 from the right to the left side, may be described as a trans- 

 verse or rotator\- growth affecting the pharvnx /// .\Vii in 



