ox THE FOltMATTOX OF 'I'HE UEP.IMIXAL LAYERS IX PETROMYZOX. 



147 



Fig8. 2Ü-22. Obli(|Uf]y transvev.sc sectiDiis tlu'on.i^h the cephalo-cervical region, 



BB X 4. 

 Fig. 20. Trausvorse .section oï an onibryo of a little niore atlvauced stage than that 



of üg. 19. 

 Fig. 21. Transverse section from a still i)lder egg. 

 Fig. 22. Transv(>rse section from a still further advanced embryo. 

 Figs. 23-20 represent obliquely transverse sections through the plane by which the 



cephalic protuberance starts up. ]jB x 4. 

 Fig. 28. Trans\ crse section of a little older embryo than that of fig. 20. 

 Figs. 24 and 25. Transverse se(;tions fi-om a still older embryo, fig. 24 being a 



more anterior one tlian fig. 25. 

 Fig. 20. Frontal section througli the cephalo-cervical region of a much further 



advanced embryo. 

 Fig. 27. Sagittal section through an embryo of an advanced stage. 



